get_endpoint should return the override

If your adapter has an endpoint_override set then this value will be
consumed by session and used in preference to whatever you give to
endpoint_filter.

This means that if you ask the adapter for the endpoint it is going to
use to query a URL you expect to get back the override because this is
where it will be sent.

Closes-Bug: #1400174
Change-Id: I707e549a4fa349d0e9a0bdac61a2573aa2e5b434
Take plugin params from ENV rather than default

The way the argparse options were being structured, if there was a
default value set on the option it would use this value as the default
and not check the environment variables.

This is wrong, we expect the environment variables to be used and the
default value to be the final fallback.

Change-Id: Ifbd68c9de329c2e0c70824ba873caa579e8e86d0
Closes-Bug: #1388076
Merge "get_endpoint should return the override"
Merge "Take plugin params from ENV rather than default"