Rename tx slave to proposal slave.

The tx slave is going to be used for other jobs that propose changes to
Gerrit. Give it a more appropriate name. This lines up with the move
from hosting this under rackspace legacy to rackspace nova.

Change-Id: I7168d62a4a0aae27d2cd2a7dc6c37595604c8251
Merge "Rename tx slave to proposal slave."