This change permit having a *partially correct* configuration file: elasticluster will just skip the cluster with an invalid configuration, printing a warning message with the explanation of all the errors found.
Raise `Invalid` if no 'provider' option is found in `cluster` section
Add some more tests on configuration file.
Rename `test` directory to `tests`
Reduce test time (mocked instances are always running)
Fix import
Cosmetic changes
Remove useless return value for mock class.
Merge branch 'master' into docker
Add `hostname` argument to AbstractClassProvider.start_instance method.