Support native and REST Salt interfaces

[rjd15372]

crust should be able to talk with Salt using either the native python library or using the salt-api REST interface.
If using the native library, crust must be run in the same host as the salt-master, otherwise it can run in any host that has network connectivity with the salt-master host.

The salt-api configuration should be provided using either the CLI options or through the .crust config file.