Implement GET /role_assignment API call

Add support for the GET /role_assignment call as a first step
to making role_assignment a first class entity.

This patch also enables v3 collection filtering to match against
attributes of entities being returned in the list, using the same
dot notation (e.g. user.id) that we already support for policy file
checking against filters.

Limitations:

- The current implementation uses the standard v3 collections wrapper
  mechanism for filtering.  Given the potential numbers of role
  assignments in a large system, this may have performance and resource
  impacts.  A future improvement would pass the filters into the
  driver layer to keep the internal assignment processing to a minimum.

- The LDAP backend is not currently supported

Implements bp get-role-assignments

Change-Id: I6ff2ea780e39d7097a88214fbb3ddee1b924c30c
Merge "Implement GET /role_assignment API call"