reorg: simplifies packstack roles

- moves all answer_file generation to a simple role: generate answers
- deletes packstack-neutron-aio, nova-aio, quantum-aio, rhos/epel roles as
  they are replaced by ksgen settings files
- multinode role now generates the packstack answer for aio and multinode jobs

Change-Id: I6d07788cd68395826efc00fe2fcb119c83c2b9aa
reorg: merge packstack aio and multinode playbook

Change-Id: Ibf65e8ffc00bca22fe37d15267d7e02e4eccff44
ksgen/readme: adds doc about yaml tags

Change-Id: Iedec9d0ed605922b5cd82d21a1d066b3395f6fef
ksgen/test: updates env test data file

Change-Id: Id4d88d003ed9c56a974b6e86a237cc3a240252f6