Add tags to pups templates (#751)

Add tags to pups templates (#751)

* Add tags to pups templates

The purpose here is to allow greater flexibility in how and where
docker images are built and run. It achieves this by breaking up
build steps into distinct run steps which can be saved along the way.
Customizable base images may then be prebuilt with as many batteries
included as possible, with zero environment setup so those images
can then be configured at a later stage.

Add the ability to run partial pups configuration:
`build`: build base image with no db - ember build.
`precompile`: precompile stage that requires postgres and redis.
`migrate`: run migration tasks.
`db`: start bundled postgres/redis, if included.

Adds a create_db script in postgres template for creating db on the fly.
Called below in unicorn run:

updates unicorn run command with 3 env flags:
CREATE_DB_ON_BOOT: if 1, creates base db schema, allows for deferral of creation.
MIGRATE_ON_BOOT: if 1, runs db:migrate - allows for deferral of db migration.
PRECOMPILE_ON_BOOT: if 1, precompiles assets (without ember build).

PRECOMPILE_ON_BOOT initially defaults to 1 in base builds (no tags).
During the `precompile` build step, this updates the default to be 0.

All other new flags default to 0 (off). With these three flags, we're now able
to ship and start a container from a base image, and it'll be able to bootstrap
a blank database.

Updates hook to start redis before_db_migrate as before_code hook
is not guaranteed to fire before migrate tasks if pups is filtered by tags.