Remove dependency of impl on onf-extensions

As preparation for removal of helium design:

- Remove dependency of openflowplugin-impl on onf-extensions and move
SalBundleService to openflowplugin-impl, as it is registered and used
only there
- Move onf extension yang models to extension-api

See also: bug 8653

Change-Id: Ib5813b5e7a115f1918d27ea116d7d49b4605799d
Signed-off-by: Tomas Slusny <tomas.slusny@pantheon.tech>