Fix mdsal-it-base karaf setup

Since we are setting up a karaf environment, we must make sure
we are not overriding opendaylight-karaf-empty, otherwise we are
not matching distribution (and not picking up its fixes,
to ARIES-1923 for example).

JIRA: ODLPARENT-209
Change-Id: I9f723ee2edd13ee4848e302c572285517a1d552b
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>