Add missing install_tempest_plugins call to upgrade-tempest
Add missing install_tempest_plugins call to upgrade-tempest

Without this call the new installation will not run plugin-specific tests.
The call is placed after configure_tempest just like it's done in tempest
devstack code.

Change-Id: Ia2a5b9cc535c7c46728eee6284a36340745e9043

openstack-dev/grenade,openstack-dev/grenade