Use new augur clades functionality

[jameshadfield]

This commit is a WIP commit to test the new functionality being introduced in augur PR 728 [1]. This allows us to simplify the nCoV workflow as we can explicitly define the attribute names used for clade membership and branch labelling.

The release of 728 will come with a new major version of augur, and this workflow's requirements should be updated accordingly.

These changes have only been tested for the "open" build, which itself is a WIP.

[1] https://github.com/nextstrain/augur/pull/728