Use config-parent

Use config-parent

This patch switches the relevant POMs to use config-parent instead of
re-specifying the appropriate Maven plugins. It also drops
yang-gen-{config,sal} from .gitignore since they are now in target.

Change-Id: I5d27111d7061cf02d55bad3173e299f289671df1
Signed-off-by: Stephen Kitt <skitt@redhat.com>