Add test for glanceclient shells

Add unittests for the following modules:
* glanceclient.shell
* glanceclient.v1.shell
* glanceclient.v1.legacy_shell
* glanceclient.v2.shell
Also add mock library to the tools/test-requires

Implements: blueprint glanceclient-shells-unittests
Change-Id: I5ec527c5efff3726932d234d7c67e08149643f89