mock __request_validity_check to speed up tests

managercli.CliCommand._request_validity_check was getting
invoked many times, and making dbus connections. This
is not particularly fast, and not very meaningful for
unit tests, so mock it out in SubManFixture.
Merge pull request #557 from candlepin/alikins/mock_dbus_for_tests

mock __request_validity_check to speed up tests