Fixed a bug where insufficient headroom is held for quota.
Fixed a bug where insufficient headroom is held for quota.

If a role has more reservation than its quota, the current quota
headroom calculation is insufficient in guaranteeing quota
allocation. See MESOS-8339.

This patch fixes the issue by computing the amount of unreserved
resources we need to hold back such that each quota role can have
its quota satisfied. Previously, we included the quota role
unallocated reservations as part of the headroom since we knew it
would be held back, but we didn't handle the case that a quota role
had more reservations than quota.

Review: https://reviews.apache.org/r/64698/

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