[ZIMBRA] Modify SMTP Relay Host

$ [sudo] su -l zimbra -c \
> "zmprov ms `zmhostname` zimbraMtaRelayHost 'my-smtp-relay-name-or-ip:port'"

To checkout:

$ [sudo] su -l zimbra -c \
> "zmprov gs `zmhostname` zimbraMtaRelayHost"