ACDEV-1174: prevent double registration when verifying installation requests.

This is a fix for ACDEV-1174. It prevents requests to /installed if the addon already has data for a tenant.

One caveat, this means uninstalling and re-installing an addon will fail, because ACE currently doesn't clean up tenant details on uninstall (it doesn't even listen for the uninstalled lifecycle even currently). I raised AC-929 about this a while ago. I'm addressing that manually for example in my confstats addon (https://bitbucket.org/sday_atlassian/confstats-connect/src/bd75cffc151bfe123ec7ac78ecc2761fdf097d11/routes/lifecycle.js?at=master#cl-8) but I can try pushing a patch through for ACE.