allow user to pass a custom config file

should help fix #46
not sure if we want a confswitch also
add documentation for GANDI_CONFIG env var
enhance ssh command usage

Allow user to add extra arguments, pass them to ssh directly
This answers use case "gandi vm ssh pb@one ps auxw"
factorize vm_ip extraction

ip extraction is required from a few locations already
refactor ssh-key wiping, add keyscan learning
add scp to iaas module
Merge branch 'config'

This is required for the script feature

Conflicts:
	gandi/cli/commands/vm.py
	gandi/cli/modules/iaas.py
add --script option for virtual machine creation

When using --script, upload script to vm (static /var/tmp/ location)
and run it, instead of spawning an ssh session.