Give the admins::parsoid group shell access to the Parsoid machine

Gabriel tried to ssh into mexia to inspect the state of a hung Parsoid
process there, but couldn't. Looks like we only had admins::roots for
the Parsoid machines, so I propose we add admins::parsoid. For now this
group only has Gabriel and myself in it.

Change-Id: Ic0bbae905d07623e3ed3a2fd4661aed9df15dff7
Merge "Give the admins::parsoid group shell access to the Parsoid machine" into production