baremetal: fix /etc/hosts generation when api_interface has dashes

Interface names with dashes can cause problems in Ansible since dashes
are replaced with underscores when referencing facts. In the baremetal
role we reference the fact for api_interface without replacing dashes
with underscores. This may result in host entries being omitted from
/etc/hosts.

This change fixes the issue.

Change-Id: I667adc7d8a7dbd20dbfa293f389e02355f8275bb
Related-Bug: #1927357