core: Rename OvfWriterTest to OvfManagerTest

Change-Id I9b5132300fb1f1fd94f771cab15efe5246dbeca8 add a bunch of tests
related to templates to this class, making the name OvfVmriterTest
incorrect.

This patch renames the test to OvfManagerTest, to better reflect what it
does.

Change-Id: I190cba03c06da688b18cc31dd65992c6d2817399
Signed-off-by: Allon Mureinik <amureini@redhat.com>