Fixing kolla's problem with ansible var precedence

default_*_interface options were introduced in order to fix problem
with *_interface vars specification made in inventory file.

This change makes ceph installation possible. Without this, ceph will
be able to use only one network: api network.

Change-Id: I516c0df385d082607238589f68e2929434d961a3
Closes-Bug: #1604129
Closes-Bug: #1586524