Add a flag to limit which resources are operated on

[ahal]

In fxci-config we added a --resources flag which allows to only generate a subset of the registered resources. For example, if you run:

tc-admin diff --resources worker_pools

Only the worker pool generation happens. This is different than --grep which generates everything, but then filters out what gets displayed. This is useful as it cuts the generation down from ~2 minutes to ~5 seconds in many cases, so it's invaluable while developing.

The problem is that because the flag is tacked on outside of tc-admin, it doesn't play nicely in some cases. For instance, the above command still requires the auth:list-clients scope (even though we aren't even generating clients) because of this line:

tc-admin/tcadmin/current/__init__.py

Line 19 in 39d7017

clients.fetch_clients(resources),

I realized that --resources needs to be implemented in tc-admin if we're going to fix things like this.

[ahal]

Once this is fixed, we should remove this hack in fxci-config:
mozilla-releng/fxci-config#232