replace baremetal_nic with api_interface

- The baremetal_nic variable was used as part of
  kolla host playbook when generating the /etc/hosts file.
  Populating the /etc/hosts is done to ensure that rabbitmq
  can reach the other nodes in the rabbitmq cluster by hostname.
- Before this change the baremetal_nic variable was used to
  determin which interface to read when mapping the hostname to
  an ip address.
- This change removes baremetal_nic and uses api_interface
  instead as the hostname used by rabbitmq should always map to the
  api_interface ip.

TrivialFix
Closes-Bug: #1618381

Change-Id: I65fe3aad58ac34b001a1f4a38641addc9fa5a1d4