Fix incorrect ca arguments for calling ceilometer client

With ceilometer configured with SSL, Horizon fail to connect to
ceilometer to load the meters(usage data) due to it passed a wrong
of ca argument for ceilometer client. Should use 'cacert' instead
of 'ca_file'.

See ceilometer client:
https://github.com/openstack/python-ceilometerclient/blob/master/ceilometerclient/v2/client.py#L53

Change-Id: I296c61119e07cd2e19079ef62054c5837fa13114
Closes-Bug: #1388764