This change adds the option to put quota on resources on a per-tenant
basis (i.e. Zuul tenants).
It adds a new top-level config structure ``tenant-resource-limits``
under which one can specify a number of tenants, each with
``max-servers``, ``max-cores``, and ``max-ram`` limits. These limits
are valid globally, i.e., for all providers. This is contrary to
currently existing provider and pool quotas, which only are consindered
for nodes of the same provider.
Change-Id: I0c0154db7d5edaa91a9fe21ebf6936e14cef4db7
ee90100852