Allow max_stacks_per_tenant configurable

[Bui Hong Ha]

By default heat's max_stacks_per_tenant is set to 100, so is other heat's quota.
For some deployment, these limitations can be over.

Please make heat's quota variables configurable through charm config variables.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.opendev.org/719608

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.opendev.org/719608
Committed: https://git.openstack.org/cgit/openstack/charm-heat/commit/?id=50d114aa1f87c7007ed3a3842507a5c2b92179a2
Submitter: Zuul
Branch: master

commit 50d114aa1f87c7007ed3a3842507a5c2b92179a2
Author: Felipe Reyes <email address hidden>
Date: Mon Apr 13 12:50:03 2020 -0400

    Add max-stacks-per-tenant config option

    This configuration option allows operators to change the default of
    how many stacks a project can have in parallel.

    This key has been available since Mitaka:
    https://docs.openstack.org/mitaka/config-reference/orchestration.html

    Closes-Bug: #1871772
    Change-Id: I4dc0008c1098a969d62ee4a8afe76de4bfb5c235