Add nose to requirements.txt

api/test_extension_driver_port_security.py imports ddt, which
fails locally running pep8 with:

    import ddt
    from nose.tools import nottest
    ModuleNotFoundError: No module named 'nose'

Adding nose to requirements.txt fixes this.

Trivialfix

Change-Id: Ie5387a10fc18ecad53dc5dda2df3dcdaf641cd2f
diff --git a/requirements.txt b/requirements.txt
index d3fa3eb..3edf7dc 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -15,6 +15,7 @@
 tempest>=17.1.0 # Apache-2.0
 tenacity>=3.2.1 # Apache-2.0
 ddt>=1.0.1 # MIT
+nose>=1.3.7 # LGPL
 testtools>=2.2.0 # MIT
 testscenarios>=0.4 # Apache-2.0/BSD
 eventlet!=0.18.3,!=0.20.1,>=0.18.2 # MIT