tools: Adds validate only argument to notifier

Adds command line argument to notifier, that executes only validation of
notification properties and exits after.

Change-Id: I2d95f2f4225652d47732c12e5e0716b7f625bbe3
Bug-Url: https://bugzilla.redhat.com/989382
Signed-off-by: Martin Perina <mperina@redhat.com>