Create auto-configuration mechanism for Lotman + Caches

[jhiemstrawisc]

Caches know how large of a disk they have to work with when the start up. They also query the Director for information about all namespaces in the federation. We should use this fact to auto-configure a fair share policy in Lotman so that lotman-enabled purges have the information they need to keep cache use balanced.

In particular, the approach to implementing this should consider that Lotman purge policies will be named entities, and so any new configuration bits should take this into account.