Replace hard-coded mail-alarm sender with a user-setable value.

Without this patch, email from the pool are sent from
noreply@${FSQN}. Some organisations setup their email so as to only
allow emails from  whitelist of addresses, so are unable to get mail
alerts.

This patch lets the administrator run:
xe pool-param-set other-config:mail-sender=${SENDER}
to choose an whitelisted address.