Move RULE_TYPE_MINIMUM_PACKET_RATE under setup method

On Wallaby, We have pinned tempest[1] to particular commit[2]
to fix lp#1963907.

https://review.rdoproject.org/r/c/rdoinfo/+/40273 pinned
neutron-tempest-plugin at master commit to fix neutron tempest
tests in wallaby.

But the tempest tests are throwing
```
AttributeError: module 'neutron_lib.services.qos.constants'
has no attribute 'RULE_TYPE_MINIMUM_PACKET_RATE'
```

On wallaby, neutron-lib=2.10.2 is used in the RDO/Tripleo
Deployment.

https://review.opendev.org/c/openstack/neutron-lib/+/816447
Moves RULE_TYPE_MINIMUM_PACKET_RATE to neutron-lib which is available
in Yoga release and not available in wallaby neutron-lib
leading to above failure.

Moving the RULE_TYPE_MINIMUM_PACKET_RATE variable to the setup
method fixes the issue.

[1]. https://review.opendev.org/c/openstack/tempest/+/822560
[2]. https://review.rdoproject.org/r/c/rdoinfo/+/40228

Related-Bug: #1963907

Signed-off-by: Chandan Kumar (raukadah) <chkumar@redhat.com>
Change-Id: If2badd6cbac13ddaf86fe81b48ebd69d2857f1b0
1 file changed