Add designate-manage path to tempest via a devstack script

So far, designate-manage was available to use in
designate-tempest-plugin tests, but its path was not well known.

This patch adds a devstack script that would set a tempest.conf setting
for the designate-manage path.

Change-Id: I26203522aaee9bb498bea9a1561eee65f6588244
4 files changed