The filter `level` was originally added in
5a71d77
for RBD level but was the wrong field name.

The correct filter `rbd_level` was later added in
1b4846a.

The clade labels were updated in #1065.
Update the clade label so that the `assign_rbd_levels` script can find
the correct basal clade.

I had considered pulling this value out as a parameter in the config YAML,
but the original commit message¹ implies that this should _not_ be
configurable.

¹ fb5f44e

The JSON output from `augur clades` was updated to separate `nodes`
and `branches` in nextstrain/augur#728 so now
the `assign_rbd_levels` script needs to parse the `branches` in order
to find the basal node.