Added multibackendency to devstack plugin

This commit allows to configure devstack to
install one or two share backends.

Two backends will have same configuration, except
share_backend_name option and group name.

NOTE: proper usage of two generic drivers
      is blocked by b_u_g 1321340

Partially implements: blueprint multibackend-installation-tempest-job

Change-Id: I4fb6fcccbf3663940e8fbb3cc842831ed62afafa
Merge "Added multibackendency to devstack plugin"