fix(flags): Expose global flags to dgraph subcommands.

[danielmai]

This updates the subcommand --help template so that flags like --config show
up in the --help output under the "Global Flags:" section.

Example:

$ dgraph alpha --help
...
Global Flags:
      --alsologtostderr                  log to standard error as well as files
      --bindall                          Use 0.0.0.0 instead of localhost to bind to all addresses on local machine. (default true)
      --block_rate int                   Block profiling rate. Must be used along with block profile_mode
      --config string                    Configuration file. Takes precedence over default values, but is overridden to values set with environment variables and flags.
      --cwd string                       Change working directory to the path specified. The parent must exist.
      --expose_trace                     Allow trace endpoint to be accessible from remote
      --log_backtrace_at traceLocation   when logging hits line file:N, emit a stack trace (default :0)
      --log_dir string                   If non-empty, write log files in this directory
      --logtostderr                      log to standard error instead of files
      --profile_mode string              Enable profiling mode, one of [cpu, mem, mutex, block]
  -v, --v Level                          log level for V logs
      --vmodule moduleSpec               comma-separated list of pattern=N settings for file-filtered logging

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This change is