Take EC2 config parameters from environment variables.

E.g., you can either have `ec2_url` in the `[cloud/*]` section,
or an environment variable named `EC2_URL`.
Mark `Abstract*` classes abstract using Python's `ABCMeta`.

This ensures that the classes cannot be instanciated unless all
abstract methods have been overridden.
Implement an `sftp` subcommand.
Pass the `-v`/`--verbose` option on to SSH and SFTP.
Implement `sftp` command

Merge branch 'master' of https://github.com/riccardomurri/elasticluster into sftp

Conflicts:
	elasticluster/conf.py
`AbstractCommend.pre_run` is not a mandatory method.