Switch to using yangtools version of mockito-configuration

This commit switches to use the artifact from yangtools repository
introduced in https://git.opendaylight.org/gerrit/#/c/4316/, completing
purge of bgpcep dependencies.

Change-Id: I000ddbb237a8a6c6cf9e9a44baa9a6dd93badc0f
Signed-off-by: Robert Varga <rovarga@cisco.com>