Streamline user.full_ha.yaml

Many variables are defined in both user.common.yaml and
user.full_ha.yaml. Remove variables from user.full_ha.yaml which are
already specified in user.common.yaml and add comments documenting
variables in user.full_ha.yaml.

Change-Id: Ida18fdbfc4c487a2988d511ae6dcedd62c715272
Closes-Bug: #1277114