Fix the use of $nagios_group.

This is again needed for puppet 3 migration, as definitions of
$nagios_group deep down in classes will not work there. Also, I fixed an
apparent bug where in general we did not set the correct
hostgroup.

The logic now is:

- If $group is explicitly declared in the resource definition, use that
- Else if $nagios_group is explicitly defined at node level, use that
- Else if $cluster is defined at node level and is not 'misc', use
  ${cluster}_$::{site} (es cache_bits_eqiad). This DOES NOT apply to
  servicegroups for individual services that have no default value
- else, leave this undefined.

This should not only retain the definitions as is almost everywhere, but
also define the servicegroup/hostgroup in a few places where it's
currently undefined.

Change-Id: I50c8d5c96b2541b1b926a98c8006ccdc665665e6
Signed-off-by: Giuseppe Lavagetto <glavagetto@wikimedia.org>