Add new Feature: Scheduler retries

If DocImpact change https://review.openstack.org/#/c/9540/ gets merged, we need to add information about new scheduler features:
Add a generic scheduler retry for build failures. Failed
build requests get casted back to scheduler for retry until
success or the maximum number of attempts is reached. The
number of attempts to make is configurable or can be
simply set to 1 to disable retries altogether.

There is an associated cost function, retry_host_cost_fn, to
de-emphasize re-scheduling to the same host.

Partially implements blueprint: scheduler-resource-race

DocImpact:

Adds a new capability to filter scheduler to enable retries of
scheduling requests.

1) New flag: scheduler_max_attempts (int) - Number of attempts to make
to schedule an instance before giving up and settting the instance to
error.
2) New filter scheduler cost function:
nova.scheduler.least_cost.retry_host_cost_fn -

Adds additional cost for retrying scheduling at a host that was
already used for a previous scheduling attempt.

retry_host_cost_fn_weight is the associated FLAG to configure
weight.