This patch is aim to fix mistralclient not pass ca_cert when user
enable ca_cert in openstack and use OpenStack CLI to call mistral
API.
* Add ca_cert parameter in plugin.py
Closes-Bug: #2120693
Change-Id: Id976d1affef7fe5a266653ed1d43650aa0bea26b
Signed-off-by: shuwcai <shuwcai@cisco.com>