Add kerberos related settings in defaults

KERBEROS_PRINCIPAL is the service principal used with:
 # k5util -v add -p

KERBEROS_KEYTAB is by default /etc/krb5.keytab

KERBEROS_KSTART_ARGS defaults to "-H 1 -l 1h" (i.e. checks
for a happy ticket and sets the ticket lifetime upon renewal)

KERBEROS_TICKET is the snf-network-kerberos.tkt file and stored
under STATE_DIR (/var/lib/snf-network).

Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>
debian: Change snf-network-dnshook hook dirs

Let snf-network-dns-hook run only after instance-add,
instance-remove, instance-modify, and instance-rename.

Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>
debian: Update control file for kerberos support

Recommend heimdal-clients and kstart debian packages since they
are needed for nsupdate to authenticate using the kerberos method.

Recommend bind9utils debian package that ships dnssec-keygen
which is needed to create the key pair needed for the bind9
authentication method.

Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>
debian: Add AUTHENTICATION_METHOD to defaults

..and let it be bind9 in order to keep backwards compatibility.

Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>