Fail early if missing root password
Fail early if missing root password

If a deployer forgets to set a user secret for the galera_root_password
variable, the deployment will make it halfway through the galera
installation and fail. This patch causes the galera role to fail very
early, which allows deployers to set the variable before partially
deploying galera nodes.

Change-Id: If0863915e6856496bded410452959e4c2ea5155d
Closes-bug: 1542440

openstack/openstack-ansible-galera_server,os-cloud/openstack-ansible-galera_server,openstack/openstack-ansible-galera_server,os-cloud/openstack-ansible-galera_server