Make sure that locks are renderable.

The expected usage is that someone would create a renderable that
returns a list of locks so that this list can be dynamic.

master/buildbot/process/buildstep.py

@@ -438,6 +438,8 @@ def startStep(self, remote):
             name=util.ascii2unicode(self.name))
         yield self.master.data.updates.startStep(self.stepid)
 
+        self.locks = yield self.build.render(self.locks)
+
         # convert all locks into their real form
         self.locks = [(self.build.builder.botmaster.getLockFromLockAccess(access), access)
                       for access in self.locks]