Use the right class name for MultiplePluginsFound
Use the right class name for MultiplePluginsFound

I8ae5369bcf5ef3aba0fa39a95d8c40cf75edb4bf renamed MultipleMatchesFound
into MultiplePluginsFound but forgot to rename the exception class in
engine.py.

Change-Id: I706f87af5e91b280ff6b84e793387e4846cc5517
Closes-Bug: #1718785

openstack/rally,openstack/rally,openstack/rally,openstack/rally