Doc about specifying domains in domains specific backends

When enabling the domain specific backends configuration, the
user needs to specify a domain, by passing as a filter in the
call or by using a domain scoped token.

Change-Id: I5b19b6d7091df474a7b68d4ffdb052efbb8e7104
Partial-Bug: 1387379
Move injection unit tests to keystone/tests/unit

The test_injection.py unit tests to the keystone/tests/unit directory.

Change-Id: Id06f059b6d475dce977865997b8f8649b15eec98
Move base64 unit tests to keystone/tests/unit dir

Move the unit tests for test_base64utils.py to keystone/tests/unit since
they are unit tests.

Change-Id: I2b68422fd3f46f9dc13ac68d96b94502765ef8a4
Increase test coverage of test_base64utils.py

This change pulls common code into a private method and increases test
coverage to 100% for test_base64utils.py.

Change-Id: I97aac1eccb92ae35e7997a95ba46e833b0d58be6
Increase test coverage of test_versions.py

This commit adds a couple unit tests to test_versions.py so that it's
test coverage is 100%.

Change-Id: If653c353953c09f6f3d5644b04e37dec5aebf196
Merge "Move base64 unit tests to keystone/tests/unit dir"
Merge "Increase test coverage of test_base64utils.py"
Merge "Doc about specifying domains in domains specific backends"
Merge "Increase test coverage of test_versions.py"
Merge "Move injection unit tests to keystone/tests/unit"