API: Fix for VmMapperTests

Adding mock for OsRepository. The sequence of the tests is
not constant, and the mock wasn't set before the test for some runs that
testMapHostId ran before the other vmMapperTests.

Change-Id: I80a76b5fea41e3a9b914bed2034f14b20fde4117
Signed-off-by: Yaniv Bronhaim <ybronhei@redhat.com>