This change adds phase documents to Treasuremap, similar to those in
airshipctl [0]. Phase documents enable airshipctl users to define phases
with document entrypoints in a declarative fashion rather than relying
on hard-coded paths to Kustomize entrypoints.
[0] https://review.opendev.org/749419
Depends-On: https://review.opendev.org/754729
Change-Id: I21dc86972c56e78b501c24dbaae6b5fb3cc2ca28