Fix syntax error in credential.py and missing __init__.py

Fix syntax error in credential.py and missing __init__.py

This patch fixes a syntax error in credential.py and resolves
the following import error:

In [1]: from quantum.plugins.cisco.extensions import _credential_view as a
---------------------------------------------------------------------------
ImportError                               Traceback (most recent call last)
/opt/stack/quantum/<ipython-input-1-ef3f11a361d0> in <module>()
----> 1 from quantum.plugins.cisco.extensions import _credential_view as a

Fixes bug 1148133

Change-Id: I22bc7152fcfec6fdc9f2638bc92d2fb1db99a484