Temp ignore use of subprocess.check_output

 It's used in plugin that only runs with python2.7,
 so not applicable.
Merge pull request #970 from candlepin/alikins/temp_fix_for_stylish

 Temp ignore use of subprocess.check_output
Allow tests to run in any TZ

StubEntitlementCertificates were setting the default
start/end dates in localtime which would cause tests
to fail if local TZ was GMT + >= 1 hour.

Certificate.is_valid() compares against UTC now which
fails against a TZ as specified above.