Hexagon: scheduled maintenance on April, 23rd 9:30

Hexagon is going to have a scheduled maintenance on April, 23rd. The
maintenance will start at 9:30. The expected downtime is about 12 hours.

During the maintenance we are going to do the following:

* Upgrade the compute node Linux to 4.2UP02.
* Upgrade the management station base OS and Cray software release.
* Apply different security patches.
* Upgrade the storage firmware.

All running jobs will terminated. The job submission system has a
reservation in place, it will not allow to start jobs which will not
be able to finish before the maintenance start.

Update 20:10 The maintenance is over. The machine is back online.