Make hostgroup params empty by default.

This requires all jobs to specify the set of hostgroup params that make
sense for that particular job. This should cut down on the problems we
see when a hostgroup param changes.

Change-Id: I9e8033b19033988f3ebb6305c599b972a5cde3b2
Devstack playbook and roles

Change-Id: Ia607920986573f338905f8c373d8fd364e9ee3e8
Merge remote-tracking branch 'gerrit/master' into dev/reorg

Change-Id: Ie8c64651db83d6dca8733f16dfee396d7f90bf77