Even if the extension is not available rally tries to list lbaas resources
to delete them. This results in a 404 exception.
This commit introduces NeutronLbaasV1Mixin that checks first if the
extension is available. If it's not it returns an empty list of
resources to delete.
All lbaas resources (only pools currently) must inherit from this mixin.
Change-Id: I94a8457c3026c1859413c867369cae49889f29e1
Closes-Bug: #1470330