Add new tempest gates for slow tests

Currently, our tempest gates (admin/Member) are skipping over slow tests (tests that take ~30+ seconds to execute), since our gates need to run under 1 hour.

This is accomplished with the following regex [0]: "(?!.*\[.*\bslow\b.*\])(^patrole_tempest_plugin\.tests\.api)".

NOTE: This regex does a negative lookahead (i.e. exclude all tests that DO have the slow tag). The regex for the "slow test" gate (described below) requires a positive lookahead (i.e. exclude all tests that DO NOT have the slow tag).

This enhancement will require doing the following:

1) Adding a new job to infra project-config under patrole.yaml
2) Passing a new argument to post_test_hook.sh called "type" (as in "test type") or something similarly appropriate, which will ensure that a different regex is executed in the "slow test" gate.
3) Ensuring that the changes to infra and post_test_hook work correctly and that only slow tests are executed in "slow test" gate.
4) 2 separate "slow test" gates will have to be implemented: one for admin and one for Member.
5) Add additional slow tests to these gates.

[0] https://github.com/openstack/patrole/blob/master/contrib/post_test_hook.sh