config: clean template, remove useless conditions

config: clean template, remove useless conditions

there is no need to have all these conditions.

for instance, assuming `mds_group_name` is set to 'mdss':

  - `if groups[mds_group_name] is defined` checks if `'mdss'` is present in `{{ groups }}`

  - `if {{ mds_group_name }} in group_names` checks if the current node is part
  the group `'mdss'`

  - `if inventory_hostname in groups.get(mds_group_name, [])` checks if
  the current node is part of the group 'mdss'

The third condition is enough to cover the need of ensuring we are
running on a mds node.

Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>