run.sh: add support for GCP_ACCOUNT

Fixes #7 

/cc @jbeda @ktintc @dlorenc 

### Tests
#### Without GCP_ACCOUNT
```
$ docker run -e GCS_BUCKET=proppy-containers --volumes-from gcloud-config proppy/docker-registry bash -c "cd /docker-registry && ./setup-configs.sh && exec bash ./run.sh"
Using credentials in /.config/gcloud/legacy_credentials/proppy@google.com/.boto
```
#### With GCP_ACCOUNT
```
$ docker run -e GCS_BUCKET=proppy-containers -e GCP_ACCOUNT=proppy@google.com --volumes-from gcloud-config proppy/docker-registry bash -c "cd /docker-registry && ./setup-configs.sh && exec bash ./run.sh"
Using credentials in /.config/gcloud/legacy_credentials/proppy@google.com/.boto
```
#### Error
```
$ docker run -e GCS_BUCKET=proppy-containers -e GCP_ACCOUNT=foo --volumes-from gcloud-config proppy/docker-registry bash -c "cd /docker-registry && ./setup-configs.sh && exec bash ./run.sh"
find: `/.config/gcloud/legacy_credentials/foo/.boto': No such file or directory
Boto credentials not found.  Either:
 - Set GCP_OAUTH2_REFRESH_TOKEN
 - Set BOTO_PATH
 - Mount gcloud credentials under /.config in the container and optionally set GCP_ACCOUNT
 - Run in GCE with a service account configured for devstorage access

docker run -e GCS_BUCKET=yet-another-docker-bucket [-e GCP_ACCOUNT='<YOUR_EMAIL>' ] [-e BOTO_PATH='/.config/gcloud/legacy_credentials/<YOUR_EMAIL>/.boto'] [-e GCP_OAUTH2_REFRESH_TOKEN=<refresh token>] -p 5000:5000 [--volumes-from gcloud-config] google/docker-registry
```