Unittests added for glance/cmd/cache_manage.py

Related to blueprint glance-cmd+unittests

Unittests for all methods, that manage cache added:
* list_cached
* list_queued
* queue_image
* delete_cached_image
* delete_all_cached_images
* delete_queued_image
* delete_all_queued_images

Unittests for several internal methods added:
catch_error; env; create_options; parse_options; lookup_command

Change-Id: Id07bbecdc9877268a514e3a594699364dcc4618f