This change adds support for model specific overlays from
templates which are rendered at deploy time.
The overlay template directory will be searched for a corresponding
template (<bundle_name>.j2). If one is found then the overlay will
be rendered using environment variables matching AMULET* or
ZAZA_TEMPLATE* as a context. The rendered overlay will be used on
top of the specified bundle at deploy time.
A default overlay is always applied "local-charm-overlay.yaml". This
overlay is only used to move the location of the charm being deployed
to a relative path so the bundle can use "charm: <charm name>" rather
than "charm: ../../../<charm name>"
e22fa7bbf5