Update gcs-oauth2-boto-plugin dependency to version 1.8

Update gcs-oauth2-boto-plugin dependency to version 1.8 to fix issue https://github.com/GoogleCloudPlatform/docker-registry/issues/17
Rename oauth2_plugin import to gcs_oauth2_boto_plugin
Add setting to boto config to enable loading credentials from GCE metadata.

In the upgrade from 1.4 to 1.8, gcs_oauth2_boto_plugin now requires GoogleCompute/service_account to exist
in the boto config before enabling the option of getting service account credentials from the GCE metadata server.
We patch the config before loading the module to make sure that option is available.
Fix invalid 'true' setting.
Merge pull request #18 from peterwald/patch-1

Update gcs-oauth2-boto-plugin dependency to version 1.8