Add CM signing directory for keystone client

Adding a signing directory for ceilometer-api
to avoid the performance issues created by keystone
client use of temp directory (default behavior when
not specified).

Addresses bug: https://bugs.launchpad.net/python-keystoneclient/+bug/1180998

Change-Id: I831780e7cd34358fd2899db00cce002ed4284a9b
Merge "Add CM signing directory for keystone client"