Combine localrc files

Use the Grenade devstack.localrc.* files as the 'default' for DevStack
by pre-pending them to the devstack-vm-gate.sh generated localrc.

devstack.localrc is now appended to the combined DevStack localrc
to still allow it to be the final config.  Normally this will be empty.

Change-Id: I18cb963c4baeeeb447bcb1436521100f1372882a
Merge "Combine localrc files"