tool: improve help output
tool: improve help output

- adds blank line in between usage summary and more detail
- better formatting for required parameters in usage
- better formatting for optional boolean parameters
- justifies the list of subcommands

Example output with subcommands:

Usage: ./build/latest/bin/kudu <command> [<args>]

<command> can be one of the following:
      fs   Operate on a local Kudu filesystem
     pbc   Operate on a PBC (protobuf container) files
  tablet   Operate on a local Kudu replica

Example output of a leaf node:

Usage: ./build/latest/bin/kudu pbc dump <path> [-oneline]

Dump a PBC (protobuf container) file
    path (path to PBC file) type: string default: ""
    -oneline (print each protobuf on a single line) type: bool default: false

Change-Id: I0b4fc2da2b03edf8ce6b9998eeabd06a4fcd216d
Reviewed-on: http://gerrit.cloudera.org:8080/4038
Reviewed-by: Adar Dembo <866ca730bfc7f544e3ebce604ceeba4879f23df0@cloudera.com>
Tested-by: Todd Lipcon <2c763d726a8c2f6afda9be7e0e56a2caa6bb5e6b@apache.org>

EvilMcJerkface/kudu,cloudera/kudu,cloudera/kudu,andrwng/kudu,cloudera/kudu,InspurUSA/kudu,cloudera/kudu,andrwng/kudu,helifu/kudu,InspurUSA/kudu,cloudera/kudu,InspurUSA/kudu,EvilMcJerkface/kudu,EvilMcJerkface/kudu,EvilMcJerkface/kudu,InspurUSA/kudu,cloudera/kudu,InspurUSA/kudu,InspurUSA/kudu,helifu/kudu,andrwng/kudu,cloudera/kudu,EvilMcJerkface/kudu,EvilMcJerkface/kudu,InspurUSA/kudu,helifu/kudu,andrwng/kudu,cloudera/kudu,andrwng/kudu,EvilMcJerkface/kudu,andrwng/kudu,helifu/kudu,helifu/kudu,helifu/kudu,andrwng/kudu,helifu/kudu,InspurUSA/kudu,cloudera/kudu,andrwng/kudu,andrwng/kudu,helifu/kudu,EvilMcJerkface/kudu,InspurUSA/kudu,EvilMcJerkface/kudu,helifu/kudu