security: only lookup hostname if _HOST substitution is required
security: only lookup hostname if _HOST substitution is required

The Kerberos principal configuration uses the special token '_HOST' to
indicate that the FQDN of the host should be specified. Previously we
would always lookup the FQDN even if the substitution was not required,
which might mean that startup would fail if there was no FQDN available,
even if no _HOST substitution was required.

Now, we only lookup the FQDN if FLAGS_principal contains the
substitution token. This provides the possibility of a workaround of
explicit principal configuration on machines with no FQDN.

Change-Id: I5de8647d6cf63ea70d880fa530fa289e8bae24fe
Reviewed-on: http://gerrit.cloudera.org:8080/7694
Tested-by: Kudu Jenkins
Reviewed-by: Alexey Serbin <864f6b82bd94e8dda64141d4274519ee66e0102e@cloudera.com>

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