Remove snapshot[1234] from mediawiki-installation dsh group

The snapshot[1234] servers in pmtpa have not been receiving MW updates
since the scap scripts were changed to python. They have been failing
due to Python version incompatibilities (Python 2.6.5 vs 2.7.3 on the
rest of the cluster).

Change-Id: I30ec5354a68df69f49aa8218f993b2353111c57c
Merge "Remove snapshot[1234] from mediawiki-installation dsh group" into production