add wikidev group to admins::pmacct

this new admins group has been added in I49c36df0a

but for the account to be actually created we need
to add the wikidev group and set the gid.

see how this is done in the other admins:: groups
as well, otherwise we get:

err: Failed to apply catalog:
Could not find dependency Group[500] for User..
at /etc/puppet/manifests/admins.pp:60

and the account does not get created due to that

RT #7368 to give Joel access to rhenium

Change-Id: Ibddb80200b0d20a33bb4e2422f5775e6ea48a1af