taken tickets should trigger spawner

[praiskup]

Even though I have this:

aws_x86_64_normal:
    max: 15
    max_starting: 5
    max_prealloc: 2
    tags:
    - copr_builder
    - arch_x86_64
    - arch_x86_64_native
    cmd_new: "/var/lib/resallocserver/resalloc_provision/vm-aws-new --arch=x86_64"
    cmd_delete: "/var/lib/resallocserver/resalloc_provision/vm-aws-delete"

And several (5+) tag-matching tickets are taken, only two builders are being started
at the same time.

[praiskup]

Well, work-around is to have max_prealloc >= max_starting which is going
to be our case anyways. So I'm marking this as an enhancement.

The challenge here is that multiple pools could match the taken ticket tags,
and we shouldn't spin up resources unnecessarily in all the pools... Perhaps
this is equivalent or similar to #24.