Remove unneeded node label parameters

Change-Id: I8c02acf3a5d735d7241ab41b285090c3d3fa0677
Make all devstack-vm jobs single-use

Use the new gearman-plugin single-use feature with all 'devstack-vm'
jobs.  This is compatible with the current devstack-gate scripts,
though once this is in place, they can be updated so that they no
longer perform relabel operations.

This is also compatible with future devstack-gate nodepool operations.

Change-Id: I6fcadb47ba5eeead3d11926019b3c041305c95f7
Merge "Remove unneeded node label parameters"
Merge "Make all devstack-vm jobs single-use"