Changes

jenkins-common.sh: fix redundant titan image build

Do not build debian-stretch-titan, when registry.osmocom.org is enabled.
The image that would be built at this point is not useful, since other
images like ttcn3-msc-test will use the debian-stretch-titan image from
the registry instead of the one that was just built locally.

Related: OS#5336
Change-Id: I7127e3ebac3a6a985c3ba50ba8c7cb8c5de978d9

jenkins-common.sh: pull registry.osmocom.org imgs

Pull debian-stretch-titan from registry.osmocom.org if the repository is
enabled. Otherwise the image gets only pulled the first time, and does
not get updated once it becomes outdated.

Fixes: OS#5336
Change-Id: I1cf998c21e4ee1f723c3b783703e339328377f3e