Resolve L2SS-188 "2021 05 27 branched from master udp recv sst client"

Closes L2SS-188

See merge request !50

L2SS-188: Renamed statistics_device -> Statistics, as it's a name, and it's a device per definition.

L2SS-251: SKA Tango images moved

Closes L2SS-251

See merge request !54

Any images missing from the artefact.skatelescope.org registry are now
configured to be downloaded from git.astron.nl:5000

This patch ensures all images available from artefact.skatelescope.org
are pinned to a specific version. This is necessary as :latest is not
available on this registry.

L2SS-251: Refer to the DOCKER_REGISTRY environment variables in .env for source images, instead of hard-coding them in the Dockerfiles

L2SS-199: Remove asyncua requirement as it's unused. Upgrade opcua lib requirement to 0.98.13.

Closes L2SS-199

See merge request !52

Resolve L2SS-230 "Add sdp sim"

Closes L2SS-230

See merge request !48

Resolve L2SS-246 "Capitalise sdp"

Closes L2SS-246

See merge request !51

L2SS-232: Add a Docker tag/push script for local images

Closes L2SS-232

See merge request !49

L2SS-225:  Fix rebuild_system_from_scratch.sh to also function on CentOS7

Closes L2SS-225

See merge request !41

L2SS-229 "Add gitpython and python-logstash-async to itango"

Closes L2SS-229

See merge request !46

L2SS-231: add a device with 20 attributes that generate random numnbers on read

Closes L2SS-231

See merge request !47