Add clients provisioning feature

The clients provisioning feature will allow create clients to be used on
tests that needs another machine to consume some content.

Also this improve the ssh module to allow specify the machine where to
run a command. Improved logging to show what machine the command was
executed (inspired by Fabric).

Finally was added tests for the new robotello.vm module and properly
added entries for added modules in the docs.
Better to set these two attributes in `__init__`, I think. At some point in the future, we may do something cool like set `provisioning_server` or `image_dir` on a per-`VirtualMachine` basis, or we may want to change the config file at runtime. Also, this module cannot be imported (and therefore linted or documented) unless the config properties listed here are set, as class attributes are evaluated at class definition time.