Data Processing: Making cluster template field required

The cluster_template field is now required in order to
create a cluster.  The user will now be prompted to choose
a cluster template if they forget to pick one.

Change-Id: If10029164aa69702da3c5255a29fdd6ddaaa3b8d
Closes-Bug: bug #1336300
Co-Authored-By: Timur Nurlygayanov <tnurlygayanov@mirantis.com>
Merge "Data Processing: Making cluster template field required"