resatpi: Partial Cluster Version Causes NPE

When creating a new cluster, providing only major or minor fields of the version
(not both) will cause crash

Change-Id: Iabc59e204a9f5bd37d6e2b41e5377aeb10d6e184
Bug-Url: https://bugzilla.redhat.com/988067
Signed-off-by: Ori Liel <oliel@redhat.com>