Changes

centos8-build: new image

Create centos8-build similar to debian-stretch-build, as preparation for
ttcn-3 tests on centos 8.

Related: OS#4564
Change-Id: Ide1e88c92b7ac301f0ea8ca110c1b2eb43f9d5ad

docker_images_require: scope of i: local

Prepare to call docker_images_require from itself, make i a local
variable.

Change-Id: I228f54f3e7431dba9e8d2a5fab0ecba3d0e6d76b

docker_images_require: build depends too

Add a function to automatically resolve the most common dependencies,
osmo-* need debian-stretch-build and ttcn3-* need debian-stretch-titan.
Do this in a shell function in jenkins-common.sh, so we can extend it in
the next patch with distribution specific dependencies (osmo-*-centos8
needs centos8-build).

Related: OS#4564
Change-Id: I98760c6dc3206529c0e267338c9f6f8e1d5be33a

ttcn3-*-test: remove implicit dependencies

Don't list debian-stretch-build and debian-stretch-titan as dependencies
anymore, now that docker_images_require finds them automatically.

Related: OS#4564
Change-Id: I3cb6b693f3dd9df84fb29c0217bb3dae454cc18f

make/Makefile: add DISTRO argument

Prepare for distro-specific osmo-* images. While at it, put each
--build-arg on a separate line for readability.

Related: OS#4564
Change-Id: I1bf1c46295035a073e440faea3696ce53e7d7985

jenkins-common.sh: support osmo-*-centos8 images

Make it possible to call the ttcn3-*/jenkins.sh scripts with:
	IMAGE_SUFFIX="master-centos8"

The existing docker_images_require lines of these jenkins.sh scripts
will then call docker_images_require with arguments like
"osmo-mgw-master-centos8". For example, from ttcn3-mgw-test/jenkins.sh:

	docker_images_require \
		"osmo-mgw-$IMAGE_SUFFIX" \
		"ttcn3-mgw-test"

Let docker_images_require build the image from
osmo-mgw-master/Dockerfile (-centos8 is cut off from the dirname) and
with DISTRO=centos8 as argument.

Collisions with the debian-stretch images are avoided by setting IMAGE
to the full image name (e.g. osmo-mgw-master-centos8).

Related: OS#4564
Change-Id: I598a262fe1a7ed4dd89e13c53e4ded103c6e3b91

osmo-*-master: support centos8

Adjust the Dockerfiles, so 'docker_images_require osmo-mgw-master-centos8'
etc. result in a centos8 based image. centos8-build already configures the
Osmocom nightly repository, so we only need to configure it for
debian-stretch.

Related: OS#4564
Change-Id: I6579748056ce0505304378b342a698b98c77fd18