Create backup of persistent stores at key times

Created on master promotion (technically just before persistence starts) by reading from the store,
and on demotion by HA subsystem by reading from local in-memory model.

Includes new BrooklynServerPaths cleaning up logic from BrooklynServerConfig, and some cleanups to the utils in BrooklynPersistenceUtils.
Relevant config keys are the *PERSISTENCE*BACKUP* options in BrooklynServerConfig.

Logging now looks like this:

     2014-11-11 13:46:06,153 INFO  Rebinding from /Users/alex/.brooklyn/brooklyn-persisted-state/data...
     2014-11-11 13:46:06,409 INFO  Rebind VanillaSoftwareProcessImpl{id=wGwL3QXG} connecting to pre-running service
     2014-11-11 13:46:06,606 INFO  Rebind complete (MASTER) in 669ms: 1 app, 2 entities, 14 locations, 0 policies, 9 enrichers, 0 feeds, 0 catalog items
    *2014-11-11 13:46:06,697 INFO  Back-up of persisted state created on promotion, in /Users/alex/.brooklyn/brooklyn-persisted-state/data/backups/2014-11-11-1346-oZyOou3V-promotion-s5kE
     2014-11-11 13:46:26,362 INFO  REST using security provider brooklyn.rest.security.provider.BrooklynUserWithRandomPasswordSecurityProvider
     2014-11-11 13:46:26,363 INFO  Allowing access to web console from localhost or with brooklyn:Ao3m5nZoe9
    *2014-11-11 13:46:46,825 INFO  Back-up of persisted state created on demotion, in /Users/alex/.brooklyn/brooklyn-persisted-state/data/backups/2014-11-11-1346-oZyOou3V-demotion-XYlN
     2014-11-11 13:46:47,045 WARN  Management node oZyOou3V detected master change, from  (?) to orZg7gai (1415713606000 / 1.05s ago) http://almacretin.local:8082/
     2014-11-11 13:46:47,059 INFO  Rebind complete (HOT_STANDBY, iteration 0) in 220ms: 1 app, 2 entities, 14 locations, 0 policies, 9 enrichers, 0 feeds, 0 catalog items

In future it would be nice to have periodic backups and some automatic clean-out support ... but for another day!