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.