CP-3657: Update alert priorities

* Reversed the meaning of mail-min-priority config key, and renamed it
mail-max-priority.

* Removed priority checking from Xapi_vmpp.

* Updated priority checking in mail-alarm.
CP-3657: Re-add alerts kept for compatibility

* Updated comments to reflect XenCenter usage.
Revert "CP-3657: Re-add alerts kept for compatibility"

This reverts commit 7c9e84b9c5910b5c38870b3133cc283688d82293.
Revert "CP-3657: Update alert priorities"

This reverts commit be1cf12042e021fb2fcd30397e4975000197cebd.
CP-3657: Update alert priorities

* Reversed the meaning of mail-min-priority config key, and renamed it
mail-max-priority.

* Removed priority checking from Xapi_vmpp.

* Updated priority checking in mail-alarm.

* Updated comments to reflect XenCenter usage.
Remove erroneous printing of messages by the CLI.

The CLI has always had the facility to print messages associated
with VMs after an operation has been performed. With the recent
reprioritisation work, this caused low priority messages to be
printed rather than high priority messages.

Signed-off-by: Jon Ludlam <jonathan.ludlam@eu.citrix.com>
Merge pull request #1098 from jonludlam/CP-3657-clearwater

CP-3657 for Clearwater