scenario manager in tempest plugin was copied from
Tempest to avoid any plugin break.
This copied version of scenario manager is using lot of
Tempest interfaces. Many of them are not stable and might
change which can break plugin.
For example - https://review.openstack.org/#/c/503875/3
This commit shrinks the scenario manager copy to keep
only required methods.
Change-Id: I2b0ce582775dcaee92edd84ce7adffe67edb67f5
a0f99c2c59