Make sure config-api brings in osgi-core

config-api is used in config-generated code, which needs osgi for
compilation. Unfortunately augmenting all current users is prohibitive,
so just make sure all users inherit osgi-core as a compile-time
dependency.

The real fix is to create a parent, which will pull osgi-core into
artifacts which actually invoke yang-jmx-plugin.

Change-Id: I24caf20ea1ab0ce32f9d259ca650f38668a8c7a5
Signed-off-by: Robert Varga <rovarga@cisco.com>