Multiplatform support for policy

Multiplatform support for policy

Add multiplatfrom support for policy-base
and policy-common docker images:
   - the pom.xml files are modified to push and pull from dockerhub instead
     of nexus repository (as nexus lacks support for manifest list)
   - for each image that is built with maven, an arch suffix is added to the
     tag. The currently supported architectures are amd64 and arm64.
   - the creation of the timestamped tag is moved from the pom.xml files to
     the docker_push_manifest.sh script
   - for each image built via the Jenkins CI jobs, the docker_push_manifest.sh
     is called. The script pushes a manifest list for each tag, plus it
     creates the timestamp tags for SNAPSHOT or STAGING images,
     depending on what job called the script (merge job, or stag job)

For using the images, the name of the manifest list is to be used (which
is the same as the current images in nexus)

Issue-ID: POLICY-1997
Change-Id: If54e7ee6ac432a999844d3584e146a90e2247323
Signed-off-by: Paul Vaduva <Paul.Vaduva@enea.com>
Signed-off-by: Cristina Pauna <cristina.pauna@enea.com>
Signed-off-by: Alexandru Avadanii <Alexandru.Avadanii@enea.com>