[neutron][drivers][ironic] FFE request - Use openstacksdk for ironic notifiers

Hello fellow openstackers,

I'd like to open an FFE request to convert the ironic events notifier from the current ironicclient to openstacksdk with the change https://review.opendev.org/682040 This requires also a bump of the versions of mock and openstacksdk in lower-constraints, included in the CR. The change has been tested in a devstack environment and passes the zuul checks.

Thank you!

Riccardo Pittau rpittau||elfosardo