Add metadata settings for needed kerberos principals

Add metadata settings for needed kerberos principals

These are only used for TLS-everywhere, and fills up the kerberos
principals that will need to be created for the certs used by the
overcloud. With this, the metadata hook will format these principals
correctly and will further pass them on to the nova metadata service.
Where they can be used if there's a plugin enabled.

bp tls-via-certmonger
bp novajoin

Change-Id: I873094bb69200052febda629fda698a7a782c031