Add boto config values to allow specifying default values for location and storage class of new buckets

Use case: some EU users do not want to create buckets in the US, but this is easy to do, as US is the default location when running `gsutil mb`. In order to not have to type `gsutil mb -l eu [-s ]` every time they create a bucket, it would be nice to be able to set those defaults in their boto config file, i.e.:

* Adding 'mb_default_storage_class' and 'mb_default_location' attributes under the '[GSUtil]' section
* When parsing arguments passed to `gsutil mb`, order of precedence for storage class and location values would be 1) explicitly passed in args, 2) default arg values from the user's boto configs

Start at the gsutil mb command's argument parsing and boto configuration handling. Trace how location and storage class options are read, then check the existing configuration documentation and tests, if present. Done means the two GSUtil defaults are recognized and explicit gsutil mb arguments take precedence.

Written by the indexing model from the issue text.