Add role assignments list support to identity v3

The assignments manager and its test class were created.
Some fake stubs were also added on the fakes.py module.

The "openstack role assignment list" command was created.

Change-Id: Iae94f4fee608ea3e09ff38961ad22edc38efb89c
Implements: blueprint roles-assignment-list
Closes-Bug: 1246310
Add support for extension list

 - Add support in the common section for extension list. This only
   supports Identity for now. Once the APIs for volume and compute
   are supported in the respective APIs, they will be added. Once
   network is added to this client, it will be added (the API already
   supports it).
 - Include extension fakes for volume and compute for pre-enablement.

Change-Id: Iebb0156a779887d2ab06488a2a27b70b56369376
Closes-Bug: #1319115
Merge "Add role assignments list support to identity v3"
Merge "Add support for extension list"