Correct CA cert argument for glanceclient

Heat need to pass the CA cert to glanceclient to load image data
during stack creation when glance api is configured with SSL.
Currently the client is passing the wrong cert key to glanceclient.
The key should be 'cacert' instead of 'ca_file'.

Change-Id: Ie542dda1354776e62507240c917c1cffbc222f17
Closes-Bug: #1384626
(cherry picked from commit fa7fd9d9882baf028efe0807840a4a8180cc9b9c)
Merge "Correct CA cert argument for glanceclient" into stable/juno