control: @config.control() decorator supports auto_enable=False

All control methods are now required to have this decorator. If
the method is to be disabled by default and enabled only at runtime,
add the argument auto_enable=False.

These changes were necessary due to backward-incompatible interface
changes in the argh library.
cloud_aws: pick private dns name from explicit instance attribute
Merge remote-tracking branch 'hnousiainen/master'