web: add /tenants route

This change adds the zuul:tenant_list to the scheduler gearman worker
to expose the list of tenants.

This change also adds the /tenants.json endpoint to the zuul-web as well as
a web interface to list tenants at / or /tenants.html.

Change-Id: Ia8edb52ec97ebe53205427c828944116eebe03b7
6 files changed