Refactor openjdk-8-jre-headless to have "server"- and "client"-specific slices

[cjdcordeiro]

can you please clarify? for armhf it seems that there's only client, while server is only available for other archs. Are you saying that, for armhf, the client should not be installed as part of the current slice?

I apologise for confusion. Yes, we build client for the armhf.
I was wondering if we can do something like:

slices:
  # The "core" slice provides a minimal, yet functional JRE8.
  core:
    essential:
      - libc6_libs
      - libgcc-s1_libs
      - libstdc++6_libs
      - zlib1g_libs
      - openjdk-8-jre-headless_client_vm
      - openjdk-8-jre-headless_server_vm

... 

and move the client and server vm contents into separate slices.

Originally posted by @vpa1977 in #150 (comment)