Add support for channel.ini file

We want to move the current [system]channel and [service]base|http_port|https_port variables out of client.ini and into a separate ini file. The reason for this is that channel/base are channel-specific, but client.ini is stored in the rootfs as part of the system-image package, so it's shared across all channels. This latter is also read-only.

The idea then is essentially to move the [service] section out of client.ini and into /etc/system-image/channel.ini, along with the build-number current contained in /etc/ubuntu-build. After this bug is closed, the client will ignore /etc/ubuntu-build.

Here is the format for /etc/system-image/channel.ini:

[service]
base: system-image.ubuntu.com
http_port: 80
https_port: 443
channel: daily
build_number: 20130819

(you can use = instead of : if you like)

client.ini will need a bit of shuffling too.

* remove [system]channel
* move [service]threads to [system]
* move [service]timeout to [system]

Note that the latter two will go away anyway once the d/l service integration is completed.