Use eqiad memcached servers in pmtpa also

Further to I5d64cec2d6, also update the twemproxy configuration so that
pmtpa servers use eqiad. Add cluster-independent twemproxy.yaml so
that the upstart configuration can be migrated to use that instead, in a
subsequent commit to the puppet repo.

Change-Id: Ib327d7131020807b01070d6b2913e226e367dd2b