Expose a new [networking]/driver option to allow overriding the
network driver.
Our use case is that we have multiple external networks and
floating IP pools in our cells environment, which Murano does not
yet allow you to select. This option allows us to move back to
Nova networking where we have code to handle this automatically.
Change-Id: Ic4e806dc72283fce6348ab1e2966ba3a618f91dc
Closes-Bug: #1617096