Allow locking nodes with mixed OSes

Allow locking nodes with mixed OSes

Instead of either not specifying an OS type/version and just getting
what's available - or requesting the same OS type/version for all nodes
in the job, allow requesting arbitrary mixes of OSes.

The path forward is going to be replacing things like:
roles:
- ['osd.0', 'mon.a']
- ['osd.1', 'osd.2]
- ['client.0']

With things like:
nodes:
- os_type: ubuntu
  os_version: '16.04'
  roles: ['osd.0', 'mon.a']
- os_type: ubuntu
  os_version: '14.04'
  roles: ['osd.1', 'osd.2]
- os_type: centos
  os_version: '7.3'
  roles: ['client.0']

Signed-off-by: Zack Cerza <zack@redhat.com>