sets default values to undef

Sets the default values of access_key_id and secret_access_key to `undef`
to ensure that they are nil when we check them in the provider
moves key usage to configure_aws method

Calls `AWS.config` with the access_key_id and secret_access_key unless
they are `nil`. If they are not provided then the aws-sdk will look for
the key in other places, as described here:

http://docs.aws.amazon.com/AWSSdkDocsRuby/latest/DeveloperGuide/ruby-dg-setup.html#set-up-creds