metadata_region in ~/.heirloom.yml not working

This PR addresses Issue https://github.com/intuit/heirloom/issues/151

Not ready to merge

@thbishop It seems this issue is occuring because trollop is configured to use a default value for the region if it isn't passed on the command line. However, later in the code https://github.com/intuit/heirloom/blob/master/lib/heirloom/cli/shared.rb#L11  of Heirloom the logic interprets that it was passed on the command line (due to the magic of trollop) so it is overriding the value from the config file with the default value.

One thought, and the easiest to fix is that we remove the default value `:default => 'us-west-1'` for the `metadata_region` option so it must either be in the config file or passed on the command line.  This could have breaking changes for anything using heirloom if they dont specify the region in the config file or on the command line. Thoughts?

` opt :metadata_region, "AWS region to store Heirloom metadata.", :type    => :string, :default => 'us-west-1'`

cc. @brettweavnet @rickmendes