Use trellis-cli for Vagrant galaxy install
Use trellis-cli for Vagrant galaxy install

This replaces the default galaxy command (`ansible-galaxy install`) with
the much better `trellis galaxy install` if trellis-cli is available on
the host machine.

With trellis-cli's command, there should be no reason to use the
annoying `SKIP_GALAXY` env var which was added to prevent Vagrant from
always installing the roles each time. `trellis galaxy install` is
efficient and quickly skips already installed roles.

pressbooks/trellis,roots/trellis,pressbooks/trellis,roots/bedrock-ansible,roots/bedrock-ansible,roots/trellis