Lower default IRONIC_VM_SPECS_RAM

Revert the default required RAM for Ironic nodes from 1024MB to 512MB.
Ironic now passes a kernel parameter to deploy ramdisks that avoids use
of tmpfs and lowers minimum memory requirements.

Change-Id: I20226c496fdd06ecfc6769a60b6d6f82887130a8
Merge "Lower default IRONIC_VM_SPECS_RAM"
Merge commit 'refs/changes/83/86083/2' of git://gold.eng.hq.xensource.com/git/github/openstack-dev/devstack into tmpbranch
Merge commit 'refs/changes/74/106774/2' of git://gold.eng.hq.xensource.com/git/github/openstack-dev/devstack into tmpbranch
Merge commit 'refs/changes/75/106775/1' of git://gold.eng.hq.xensource.com/git/github/openstack-dev/devstack into tmpbranch
Merge commit 'refs/changes/45/107345/1' of git://gold.eng.hq.xensource.com/git/github/openstack-dev/devstack into tmpbranch