Use updated metadata location for nova.baremetal.arch.

boot-stack was looking for a config value in the metadata
at 'nova.bm_arch', but it was recently moved to 'nova.baremetal.arch'.

Change-Id: I93447a6fa1d9be0b3278393141ab207a840ee6d8
Workaround current pbr broken-ness.

To be removed once pbr version 0.5.10 hits pypi.

Change-Id: I1efad2ab9daf509716625b4ebe1182119025641c
Merge "Workaround current pbr broken-ness."
Merge "Use updated metadata location for nova.baremetal.arch."