Devstack scripts update

1. Do not create Murano users and endpoints if keystone is not installed.
2. Install 'python-mysqldb' package as it is required by 'murano-manage db-sync'
   This package is installed by nova, neutron, keystone. In case these
   components are not selected the package is missed.

Change-Id: If3c0bdc761a3280aee522672f4ce2776fb89e126
Merge "Devstack scripts update"