config: document/warn where/when env overrides cfg

Removed StarClusterConfig.get_aws_credentials in favor of only loading
[aws info] settings from the environment once per load() call.
Refactored StarClusterConfig.get_aws_from_environ to
get_settings_from_env which now accepts an arbitrary dictionary/list of
settings to load from the environment.

Documented the fact that StarCluster will override any settings from
[aws info] that are defined in the environment. Also documented that it
logs a warning when it uses settings from the environment.

closes gh-244