Changes

osmo-uecups-master: make osmocom user avilable

Bring back the osmocom user that was removed earlier to fix a UID
conflict with the build user.

The osmocom user is being used after all, the TTCN-3 PGW tests use it to
start commands as "osmocom" user in osmo-uecups-master. This lead to e.g.
PGW_Tests.TC_createSession_ping4 failing.

Rename the user instead of adding a new one, so it can write log files
to the log dir as the same UID as the user that runs docker.

Fixes: 94f8a8 ("osmo-uecups-master: remove useradd")
Related: OS#6057
Change-Id: I2b29d7fedcf3e0a4c17bfbad16077098f94c529b

debian-bullseye-titan-*/Dockerfile: Replace inetutils-ping with iputils-ping

iCurrently we install inetutils-ping in some docker images and
iputils-ping in others; let's make it more uniform to avoid running
different implementations in different images.
The iputils-ping provides further linux-specific features, such as the
"-I interface" param, which is used by some images, so let's pick that
one everywhere.

Change-Id: If11a1c2ae1115b585b6a44a45e9569c16856d8c4