We do have a variable `rabbitmq_erlang_version_spec` which is applicable only for Debian, while for EL the variable is only private. This patch does some alignment on things: * spec is replaced with just a version, as we don't need ternary logic anymore. * defenition of erlang version is unified across distros Change-Id: Ic72212fa2d6df55f2f60d1d38c48310765442e51