Modify test_requirements.txt to fetch grizzly version of Horizon to run the tests

Modify test_requirements.txt to fetch grizzly version of Horizon to run the tests

Change-Id: I7249db0d2e5e02b12cd8707ea961391e1f5978f0
Signed-off-by: Rosario Di Somma <rosario.disomma@dreamhost.com>