Add a zuul client

Add a command line client called 'zuul' that supports one command
to start with: 'enqueue'.  It allows an operator (one with access
to the gearman server) to enqueue an arbitrary change in a specified
pipeline.  It uses gearman to communicate with the Zuul server, which
now has an added RPC listener component to answer such requests via
gearman.

Add tests for the client RPC interface.

Raise an exception if a Gerrit query does not produce a change.  Unlike
events from Gerrit, user (or admin) submitted events over the RPC bus
are more likely to reference invalid changes.  To validate those, the
Gerrit trigger will raise an exception (and remove from its cache) changes
which prove to be invalid.

Change-Id: Ife07683a736c15f4db44a0f9881f3f71b78716b2
Merge "Add a zuul client"