dissolve mediawiki::pybal_check into mediawiki::users

mediawiki::pybal_check sets up the user account and SSH key used by PyBal for
its SSH-based server health check. Strictly speaking, it is only required on
the web appservers that are behind LVS. By moving it to mediawiki::users, it
will be applied on all app servers. I think this is a boon: simplicity /
uniformity is desirable in this case.

Change-Id: I05c3830fd0d8270bc8ef5356c5816be4b32647a8
Merge "dissolve mediawiki::pybal_check into mediawiki::users" into production