Code review comment for ~ltrager/maas:lp1702567

This has been a problem for awhile when using make package-dev. I've fixed it previously by manually adding the sym link between from /usr/lib/python3/dist-packages/metadataserver/user_data/templates/snippets/maas_api_helper.py and /usr/lib/python3/dist-packages/provisioningserver/refresh/maas_api_helper.py. This fix survives reinstalls.

When I went to rebuild my environment last week for Bionic I ran into this issue again and was able to come up with this patch to actually fix it.