use psycopg2 binary instead of source package

this change replaces psycopg2 with psycopg2-binary
to avoid requiring the build depencyies for the postgres client.
instead the tests now use the prebuilt binary wheel.

Change-Id: Id6d920508b9ee9597a51616e66c47d4cb7f48ca0

test-requirements.txt

@@ -8,7 +8,7 @@ coverage!=4.4,>=4.0 # Apache-2.0
 ddt>=1.2.1 # MIT
 fixtures>=3.0.0 # Apache-2.0/BSD
 mock>=3.0.0 # BSD
-psycopg2>=2.8 # LGPL/ZPL
+psycopg2-binary>=2.8 # LGPL/ZPL
 PyMySQL>=0.8.0 # MIT License
 python-barbicanclient>=4.5.2 # Apache-2.0
 python-ironicclient>=3.0.0 # Apache-2.0