MANILA_MULTI_BACKEND has been deprecated for five years now, we should remove
it from our code base.
This variable was removed from the settings scripts along with:
MANILA_BACKEND1_CONFIG_GROUP_NAME and MANILA_SHARE_BACKEND1_NAME;
MANILA_BACKEND2_CONFIG_GROUP_NAME and MANILA_SHARE_BACKEND2_NAME.
Because they work in the same context.
Instead of them, the already implemented and in use,
MANILA_ENABLED_BACKENDS variable was placed to garantee the successful
back-end setup. The same replacement was made in the contribution
samples scripts.
Apart from this, we avoid configuring generic1 and generic2 if
another backend/s are selected.
Closes-Bug: #1898791
Closes-Bug: #1878477
Change-Id: I67036a65da9255694a00a9c8d56cfdefbdf23c05
be794fb168