tests: Add Patch class for fixture monkey patching

MonkeyPatch decorator is useful when you want to monkey patch some
modules during one test and use different or no monkey patcing during
other tests.  However, when monkey patching all methods in a test case
class, using a decorator is tedious, leads to duplicate code and hamper
readability.

This patch adds a Patch object for monkey patching during all tests in a
test fixture. New tests cover both MonkeyPatch decorator and new Patch
object and serve as example code.

Change-Id: Idd419377be951635fe7d10cb21c0d1195e88cf8d
Signed-off-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/19881
Reviewed-by: Allon Mureinik <amureini@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>