[deployment_handler] Fix apt-cache to get installed version

In case multiple versions of a package exist on the repo, the old method using
apt-cache show opendaylight returned everything available rather than what was
actually installed. Using apt-cache policy opendaylight is more appropriate.

Change-Id: I8068c18ab25ca532988f24277fd48a4c42647891
Signed-off-by: Ciprian Barbu <ciprian.barbu@enea.com>