The setting of Q_USE_PROVIDERNET_FOR_PUBLIC was added to
multitenancy job in project-config commit
7d1e2127a2b055640d90808347ae5b133df83847, so no need to set it in
the plugin settings anymore.
Multitenancy devstack setup docs are updated too.
Change-Id: I165bfc8051b9cf3fc80f299c8215712089003f3a