Changes

debian-stretch-obs-latest: new container

Move common code from osmo-*-latest to debian-stretch-obs-latest:
* add the OBS repository key
* add the OBS repository with "latest" feed
* install telnet
* invalidate the cache once the repository is updated

This is in preparation to make osmo-*-latest work with centos8,
preventing the following issues we would have without this refactoring:
* avoid need to read the repository index of both debian-stretch and
  centos8, and to invalidate the cache if either of them changes
  (instead of just the relevant one). This is currently a problem in
  osmo-*-master/Dockerfile, Id69f3eaf37f823bac0947366f27a25edd277a4cb
  will fix it there.
* no need to add boilerplate code to install the centos8 repository to
  each osmo-*-latest/Dockerfile

While at it, also remove deprecated MAINTAINER lines.

Related: OS#5049
Related: https://docs.docker.com/engine/reference/builder/#maintainer-deprecated
Change-Id: I443d52eba416ea298cc5421e280ad2f4ca59283b

osmo-*-latest: support centos8

Add a new centos8-obs-latest container with shared code (just like
debian-stretch-obs-latest) and adjust the package install blocks in
osmo-*-latest.

Related: OS#5049
Change-Id: I274b31d4903df07e99fd434af25bdb72991f6923

osmo-*-master: move common code to *-build

Move common code from osmo-*-master to debian-stretch-build and
centos8-build:
* add the OBS repository key (debian only)
* add the OBS repository with "nightly" feed (debian only, already done
  for centos8)
* install telnet
* invalidate the cache once the repository is updated

While at it, remove deprecated MAINTAINER lines.

This fixes the need to read both the repository index of debian-stretch
and centos8 and having to invalidate the cache if either of them changes
(instead of reading just the relevant index and invalidating the cache
only if the relevant one changes).

A similar change was done with osmo-*-latest in
I443d52eba416ea298cc5421e280ad2f4ca59283b.

Related: OS#5049
Related: https://docs.docker.com/engine/reference/builder/#maintainer-deprecated
Change-Id: Id69f3eaf37f823bac0947366f27a25edd277a4cb