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