Add back our ability to exclude specific sites from tree builders in the workflow with `tree_exclude_sites`

[huddlej]

Current Behavior

@joverlee521 noted that our workflow configuration includes references to sites to exclude from tree building, but the workflow doesn't use them.

This is a bug in the tree rule, since defining the tree_exclude_sites parameter should add --exclude-sites config/{lineage}/{segment}/exclude-sites.txt to the augur tree command. We only have those "exclude sites" files for HA and only the H1N1pdm file has anything in it. The loss of that functionality must be related to the refactored workflow.

Possible solution

We should update the tree rule in the core workflow rules to check for the tree_exclude_sites config and create appropriate augur tree argument for that file if it is provided.