Add support for cert based access type

This patch adds support for cert based access type in Manila.
It introduces a new 'access_type' called 'cert' in access-allow API.
'access_to' argument passed by the user is the certificate's CN
(common name) to which access is allowed by the backend.

Per the Manila IRC discussions, the cert based trust setup between
instances and backend is out-of-band to Manila. See the log @
http://eavesdrop.openstack.org/meetings/manila/2014/manila.2014-07-24-15.01.log.html
(15:27:19 onwards)

Partially-implements blueprint cert-based-access-type

Change-Id: Ib48430deccece66a740ce05471db70eaadf634b3
Merge "Add support for cert based access type"