Revert "990195: remove subscribe options"

This reverts commit 1a9884877bf833225ea89bd7ff01ee51a5687ee7.

Add "subscribe" and "unsubscribe" commands back, at least
till we get to the next major version.
Subscribe/unsubscribe mirror attach/remove tests

manangercli tests for subscribe/unsubscribe are not
sub classes of attach/remove tests.

Add test cases for RemoveCommand
Merge pull request #750 from candlepin/alikins/990195_revert_remove_subscribe

Alikins/990195 revert remove subscribe