* provider/openstack: Rename provider to loadbalancer_provider
This commit renames provider to loadbalancer_provider in the
openstack_lb_loadbalancer_v2 resource.
It also changes security_group_ids to Computed so default
security groups are added to the state correctly.
* provider/openstack: Switch to a deprecation path for loadbalancer provider
This commit switches to using a deprecation path for removal of the
previous "provider" argument in favor of the new "loadbalancer_provider".