Allow dropping units on the root container of a machine.
Merge pull request #529 from jaycee/place-on-root-container

Allow dropping units on the root container of a machine.

The changes to block dropping unplaced units on deleted machines broke the check for dropping on an existing container; this corrects that check so dropping on a not deleted root container works.
Remove the provider test results per #1339751
Disable containerization when not supported.
Merge pull request #531 from frankban/disable-containers

Disable containerization when not supported.

Implement an infrastructure to dynamically disable
containers support in the machine view based on the
environment's provider type.

A list of supported containers is defined at
environment level, and used to check containerization
support.

Avoid hard-coding the list of container types in the
create machine view.

Update the service unit token to accept a list of
available container types.
Also removed the env from the service usint token:
it doesn't appear to be used at all.

This branch also includes some drive-by fixes to
methods' docstrings and comments.
Merge pull request #530 from mitechie/hide-providers

Remove the provider test results per #1339751

Removing code for now until new charm testing lands and we'll probably want to revisit this once the charm testing comes back up anyway.