Add tempest-slow-py3 job definition
Add tempest-slow-py3 job definition

This job is based on tempest-slow job but it runs
on Python 3.

This patch also removes tempest-slow job from gate pipeline.
Running only tempest-slow-py3 job there is sufficient.
To keep py2 coverage tempest-slow job will be still run
in check queue together with tempest-slow-py3.

Change-Id: I9fad35c6327fac30a25e940f7fec8515995c68c8

masayukig/tempest,openstack/tempest,openstack/tempest,cisco-openstack/tempest,masayukig/tempest,cisco-openstack/tempest