swift: switch eqiad to tempauth backend

Use tempauth for the new eqiad cluster as swauth for a single account is
a bit of an overkill -- plus, swauth has been split from core Swift in
favor of keystone.

The changes are as minimal as possible considering the abstractions in
place for Swift; this adds an  auth_backend parameter to
swift::proxy::config which provides the options "swauth" (the current
one) and "tempauth".

While at it, switch eqiad to mw:media instead of mw:thumb, as it's much
more than thumbs for quite a while now.

Change-Id: I2ce1e8d14c2714f53d7e3d1787304826f546fb6e