Use parsed details from .gitreview for check-repo-links script

The 'linting' script makakes assumptions on the URL for the upstream project, but we can parse it from the .gitreview.  Enhance it to make it more resilient.  From #217:

```
$ REPO=$(grep "^project=.*" .gitreview | awk -F'=' '{print $2}')
$ echo $REPO
openstack/charm-octavia.git
$ echo ${REPO%.*}
openstack/charm-octavia
```