Document tempest APIs which plugins may use
Document tempest APIs which plugins may use

Plugin authors can have a hard time determining which APIs are acceptable
to use, and which are not. We should provide an explicit list on the
plugin interface doc page.

Change-Id: I475a8ccc568077a4824fda0ddc20606a0ffabbc7

Juniper/tempest,cisco-openstack/tempest,masayukig/tempest,Juniper/tempest,vedujoshi/tempest,Tesora/tesora-tempest,openstack/tempest,bigswitch/tempest,sebrandon1/tempest,sebrandon1/tempest,vedujoshi/tempest,openstack/tempest,cisco-openstack/tempest,bigswitch/tempest,masayukig/tempest,Tesora/tesora-tempest