Fix date formatting for BSD

and os x :)
system tests - postpone system unregister

Tests failing due to premature system cleaning
system tests - skip subscription-manager release --set

Since BZ 818205 it's harder to test this without real manifest.
system tests - the usage of packagegroup info changed a bit

uuid is used as identifier instead of name
system tests - don't throw away test results

The output should be handled correctly in test_own_cmd_success.
Throwing the output out meant we didn't see the test results, just
info about some failing tests in summary.