maas-rack-controller package needs updated description

[Mark Shuttleworth]

This package description should be updated to:

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The MAAS rack controller provides highly available, fast and local broadcast services to the machines provisioned by MAAS. There should be a MAAS rack controller attached to each fabric (which is a set of trunked switches). You can arrange for multiple rack controllers that are attached to these physical networks, and MAAS will then be highly available, with secondary rack controllers automatically stepping to provide these services to a rack if the primary rack controller fails.

A common configuration is to have a rack controller in each rack, with secondary network interfaces on one or two other racks for high availability redundancy.