Global flags must appear before the backend subcommand in the
versitygw CLI. Previously VGW_ARGS was appended after the backend,
causing global flags to be silently ignored.
Reorder argument assembly to: VGW_ARGS <backend> VGW_BACKEND_ARG VGW_BACKEND_ARGS
Fixes#2082
Using Docker ENTRYPOINT should allow for configuration of running
versitygw within Docker container similar to how the systemd
service is setup with environment variables.
This also adds the backends azure and plugin to the acceptable
backend options for both docker and systemd.
Fixes#1335