Feature 10974: Add juju instantiation params.

Instantiation paramaters are added using
a bundle overlay.

Bundle overlay is passed to juju library at the
moment of deployment.

Bundle overlay is a YAML file created at instantiation.

We check that all the applications in overlay already
exist in original bundle

Change-Id: Idbc7d2bc02915a1023e213e26a01531d93f24798
Signed-off-by: Patricia Reinoso <patricia.reinoso@canonical.com>