ua fails if any package removed from server

[Impact]
This bug causes enabling services during a `ua fix` operation to fail.
That means that any user that is attached to a UA contract, but needs a particular service to fix a USN or CVE could run into this bug if they try to use `ua fix` to fix that USN/CVE.

In practice, this probably mostly affects xenial users who are attached to a UA contract, but don't have `esm-infra` enabled. For them, many `ua fix` attempts will fail because they attempt to enable `esm-infra`.

The bug occurred because we are re-using a function to `enable` that has a required argument that isn't checked by mypy. When this new required argument got added to the function, we failed to update this callsite. That happened because the args are currently passed via an argparse Namespace object and so the discrepancy wasn't captured by mypy.

The fix is to pass the required argument.

[Test Plan]
To Reproduce:
```
lxc launch ubuntu-daily:xenial x-1969809
lxc exec x-1969809 -- apt update
lxc exec x-1969809 -- apt upgrade
lxc exec x-1969809 -- apt install apache2
lxc exec x-1969809 -- ua attach --no-auto-enable $YOUR_TOKEN
lxc exec x-1969809 -- ua fix usn-4994-2
# respond with "e" to the prompt
```
You should see "Unexpected error(s) occurred."

To see that release 27.9 of ubuntu-advantage-tools fixes the problem, you can use the build in `ppa:ua-client/staging` for now (or once it is in -proposed, just enable proposed).

```
lxc launch ubuntu-daily:xenial x-1969809
lxc exec x-1969809 -- add-apt-repository ppa:ua-client/staging
lxc exec x-1969809 -- apt update
lxc exec x-1969809 -- apt upgrade
lxc exec x-1969809 -- apt install apache2
lxc exec x-1969809 -- ua attach --no-auto-enable $YOUR_TOKEN
lxc exec x-1969809 -- ua fix usn-4994-2
# respond with "e" to the prompt
```
You should see "USN-4994-2 is resolved."

[Where problems could occur]
Because the fix just adds the required argument, similar problems could occur in the future if we add a new required argument (or rename an argument, etc) and again forget to update this callsite.

If our fix of passing the required arg is wrong somehow (e.g. mispelled), then this same bug will continue to occur.

If our fix is egregiously wrong (e.g. invalid python), then all `ua fix` attempts could fail, not just those that require a service enabled in the middle.

[Other Info]

In the future, we should move away from re-using the cli action functions with Namespace args for this functionality. Instead we should use functions with mypy-type-checked arguments. We will likely do this soon as a refactor of `ua fix` is on our roadmap for this cycle.

[Original Description]

I'm using the ua tool (version 27.7~16.04.1). I'm unable to enable the esm-infra service because a couple of the packages that I have installed have been completely removed from the launchpad server. Specifically:

deb http://ppa.launchpad.net/jonathonf/ffmpeg-3/ubuntu xenial main
deb http://ppa.launchpad.net/jonathonf/python-3.6/ubuntu xenial main

When I remove these package definitions from /etc/apt/sources.list.d/ the command succeeds.

I've attached a log file that includes this stack trace.

Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/uaclient/cli.py", line 1629, in wrapper
    return func(*args, **kwargs)
  File "/usr/lib/python3/dist-packages/uaclient/cli.py", line 1725, in main
    return args.action(args, cfg=cfg)
  File "/usr/lib/python3/dist-packages/uaclient/cli.py", line 480, in action_fix
    fix_status = security.fix_security_issue_id(cfg, args.security_issue)
  File "/usr/lib/python3/dist-packages/uaclient/security.py", line 607, in fix_security_issue_id
    usn_released_pkgs=usn_released_pkgs,
  File "/usr/lib/python3/dist-packages/uaclient/security.py", line 982, in prompt_for_affected_packages
    num_pkgs=count,
  File "/usr/lib/python3/dist-packages/uaclient/security.py", line 872, in _handle_released_package_fixes
    cfg, binary_pkgs, pocket
  File "/usr/lib/python3/dist-packages/uaclient/security.py", line 1223, in upgrade_packages_and_attach
    if not _check_subscription_for_required_service(pocket, cfg):
  File "/usr/lib/python3/dist-packages/uaclient/security.py", line 1139, in _check_subscription_for_required_service
    if _prompt_for_enable(cfg, ent.name):
  File "/usr/lib/python3/dist-packages/uaclient/security.py", line 1118, in _prompt_for_enable
    cfg,
  File "/usr/lib/python3/dist-packages/uaclient/cli.py", line 200, in new_f
    if cmd_args.format == "json" and not cmd_args.assume_yes:
AttributeError: 'Namespace' object has no attribute 'format'