Limit powerful builders use per-user

[praiskup]

We have these two:

# Maximum number of concurrent build workers per architecture and owner.  For
# example, give at most 15 ppc64le and 10 s390x machines to one copr owner:
#build_max_workers_arch_per_owner=ppc64le=15,s390x=10

# Maximum number of concurrently running tasks per a build tag.
#builds_max_workers_tag=Power9=5,Power8=10

But neither of them allows us to limit the number of powerful builders allocated to
a single project. We should have something like:

# Maximum number of concurrent build workers tag-set and owner.
#build_max_workers_tag_per_owner=on_demand_powerful+aarch64=2

While on it, I think we could enhance the build_max_workers_tag to accept + sign as well.