Tidies for readability and formatting in SshMachineLocation and related classes
Adds KeyTransformingLoadingCache to brooklyn-utils
Cache and reuse SshTools in SshMachineLocation.

* Uses LoadingCache to create, store and expire SshTool instances
* Properties in REUSABLE_SSH_PROPS are removed from Map given to
  connectSsh via BasicPool supplier.
Increases RedisStoreImpl sensor poll period to five seconds
Log exceptions thrown when closing SshTool pools.
Fix Cassandra fabric setting seeds
Reduce excessive cassandra-seed logging

- CassandraNode was repeatedly publishing serviceUp, causing log.debug
  every second
- Don't log on fabric stopping (otherwise complains no seeds, with
  scary threat of split-brain)
Merge pull request #1174 from sjcorbett/ssh-tool-reuse

SshMachineLocation reuses SshTools
Merge pull request #1176 from aledsage/fix/CassandraFabric-setting-seeds

Fix setting seeds in CassandraFabric
For cassandra rebind
Persist EntityRef in xml-serialization (WIP)

- Instead of replacing the entity with a string, and storing which
  fields/config/attributes this had been done to, we persist an
  EntityRef or LocationRef. On deserialisation this can be replaced
  with an EntityProxy.
- TODO: at deserialisation of the mementos, we don't have all the
  entities yet so can't crate all the proxies. E.g. if the first file
  references the second, then we can't deserialise the first's EntityRef
  into an EntityProxy.
AstyanaxSample to use different columnFamilyNames
Remove use of MementoTransformer (in persistence)
Persist completed attributeWhenReady config

- currently it's belt and braces (!)
-- MementoGenerators swaps the task for the completed val or null
-- XmlMementoSerializer has a converter for Task
Expand machine-rebind testing
Cleanup XmlMementoSerializer
Testing SshMachineLocation persistence

- and improve logging
Add alwaysRun to FixedListMachineProvisioningLocationTest.setUp
Add CassandraClusterRebindIntegrationTest
Extracted readMemento into separate method and expanded mutex scope