JCLOUDS-549: Fix NPE in LoginCredentials.toString

- also deprecates LoginCredentails.get(Password|PrivateKey)
- use getOptionalPassword and getOptionalPrivateKey instead
JCLOUDS-622: add Strings2.toString(InputSupplier)

We did not deprecate this in 1.7.x as previously thought.
JCLOUDS-622: Remove last vestiges of InputSupplier

Guava 16 deprecated InputSupplier and a future release will remove it.