Add support for accessing metadata in swarm templates (docker-archive…

…#412)

Signed-off-by: David Chung <david.chung@docker.com>

Metadata SPI (docker-archive#396)

Signed-off-by: David Chung <david.chung@docker.com>

Template improvements (docker-archive#390)

Signed-off-by: David Chung <david.chung@docker.com>

Swarm flavor revamp (docker-archive#376)

Signed-off-by: David Chung <david.chung@docker.com>

Template package cleanup + more tests (docker-archive#366)

Signed-off-by: David Chung <david.chung@docker.com>

adding SL support for terraform instance plugin (docker-archive#321)

Signed-off-by: Lin Sun <linsun@us.ibm.com>