feature: Pass mechanism_drivers from neutron-api suboridinate charms

[Jorge Niedbalski]

Currently the mechanism_drivers option is enforced to openvswitch,l2population,
but we have others to be supported as well, so this should be configurable even
if this is being handled by a subordinate charm.

# Example: mechanism_drivers = openvswitch,mlnx
# Example: mechanism_drivers = arista
# Example: mechanism_drivers = openvswitch,cisco_nexus,logger
# Example: mechanism_drivers = openvswitch,brocade
# Example: mechanism_drivers = linuxbridge,brocade

This bug tracks that implementation.

[Felipe Reyes (freyes)]

In general subordinates should be able to manipulate ml2_config.ini, SubordinateConfigContext helps to inject new parameters, but existing one cannot be modified.

[Nobuto Murata (nobuto)]

This feature is required for SR-IOV(sriovnicswitch mechanism_drivers) as well.

[James Page (james-page)]

I'd like to review this as part of the work to implement SR-IOV support - it would seem likely that we need some sort of capability to support configuration of mechanism drivers, but I would rather that is a charm function, rather than end user supplied configuration.

[James Page (james-page)]

My expectation is that the first neutron-api plugin subordinate that needs this feature will provide the implementation.