The load of plugins will eventually be moved to libosmo-gprs
implementation, and path will be passed as a parameter. Once it's moved
inside libosmo-gprs, it will be more strict on load failures, which can
cause internally if the path doesn't exist (unless NULL is passed).
Hence, add a VTY config to allow configuring the right path, and have it
disabled by default.
Change-Id: I4f965c7afafa193f4d7486750dd3e43cca22bb65