Installers should identify when running in a container and start better

When running a Docker container based on a systemd-based OS (such as Ubuntu 16.04 or Centos 7), installing the Server RPM/DEB package doesn't successfully start the server. This is mostly because the init scripts try to run systemd commands like "systemctl", and systemd basically doesn't work in containers. However, we could probably identify such environments and fall back to triggering /opt/couchbase/bin/couchbase-server directly.