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

cheng--zhang/xen-api,jjd27/xen-api,agimofcarmen/xen-api,cheng--zhang/xen-api,ravippandey/xen-api,robertbreker/xen-api,anoobs/xen-api,Frezzle/xen-api,cheng--zhang/xen-api,simonjbeaumont/xen-api,srowe/xen-api,robertbreker/xen-api,vasilenkomike/xen-api,ravippandey/xen-api,cheng-z/xen-api,thomassa/xen-api,euanh/xen-api,koushikcgit/xen-api,koushikcgit/xen-api,cheng-z/xen-api,simonjbeaumont/xen-api,ravippandey/xen-api,rafalmiel/xen-api,salvocambria/xen-api,guard163/xen-api,rafalmiel/xen-api,cheng-z/xen-api,thomassa/xen-api,rafalmiel/xen-api,agimofcarmen/xen-api,cheng--zhang/xen-api,agimofcarmen/xen-api,cheng-z/xen-api,agimofcarmen/xen-api,jjd27/xen-api,djs55/xen-api,salvocambria/xen-api,guard163/xen-api,koushikcgit/xen-api,djs55/xen-api,srowe/xen-api,vasilenkomike/xen-api,simonjbeaumont/xen-api,cheng--zhang/xen-api,simonjbeaumont/xen-api,vasilenkomike/xen-api,guard163/xen-api,guard163/xen-api,srowe/xen-api,cheng-z/xen-api,anoobs/xen-api,vasilenkomike/xen-api,salvocambria/xen-api,salvocambria/xen-api,agimofcarmen/xen-api,rafalmiel/xen-api,thomassa/xen-api,cheng--zhang/xen-api,anoobs/xen-api,thomassa/xen-api,jjd27/xen-api,cheng--zhang/xen-api,guard163/xen-api,robertbreker/xen-api,Frezzle/xen-api,rafalmiel/xen-api,euanh/xen-api,anoobs/xen-api,srowe/xen-api,Frezzle/xen-api,djs55/xen-api,rafalmiel/xen-api,jjd27/xen-api,koushikcgit/xen-api,huizh/xen-api,simonjbeaumont/xen-api,srowe/xen-api,salvocambria/xen-api,koushikcgit/xen-api,vasilenkomike/xen-api,guard163/xen-api,cheng-z/xen-api,jjd27/xen-api,huizh/xen-api,thomassa/xen-api,jjd27/xen-api,anoobs/xen-api,anoobs/xen-api,euanh/xen-api,robertbreker/xen-api,Frezzle/xen-api,huizh/xen-api,huizh/xen-api,Frezzle/xen-api,ravippandey/xen-api,ravippandey/xen-api,djs55/xen-api,ravippandey/xen-api,vasilenkomike/xen-api,euanh/xen-api,koushikcgit/xen-api,djs55/xen-api,robertbreker/xen-api,huizh/xen-api,huizh/xen-api,salvocambria/xen-api,agimofcarmen/xen-api,euanh/xen-api,cheng-z/xen-api,euanh/xen-api,djs55/xen-api,thomassa/xen-api,simonjbeaumont/xen-api,robertbreker/xen-api