Switch config to use Ironic driver from Nova's tree

Change the Nova config to use the IronicDriver from the Nova tree,
so that the copy of this driver in Ironic's tree may be deleted.

Also removes the unneeded [ironic] "sql_connection" config parameter
which was cruft copied from the nova_bm configuration in commit
06fb29c66124b6c753fdd262eb262043b4551298

Change-Id: I35473b54d760bfa7395decbe6b086f64db60ab10
Merge "Switch config to use Ironic driver from Nova's tree"