packaging: enable units testing

- added %check in the spec file, calling make check
- added check-local target in src/ovf where a unit test
  already existed
- cleaned up ovfenvelopetest.py
- avoid syntax check on generated file ovfenvelope.py

Change-Id: Ic4a3988c7067ece7f6ea60834182a2585a2f7545
Signed-off-by: Sandro Bonazzola <sbonazzo@redhat.com>