[tools] improve bkctl help message

[sijie]

Descriptions of the changes in this PR:

Motivation

Currently bin/bktcl help will output all command groups all together.
It is a bit hard to tell what command groups to use, especially some
commands are used for table service.

Changes

Introduce category in the cli spec to define which category that a
command group belongs to. So when it prints out the help message, it
can use that information to categorize the command groups together
to provide better user experience.

Results

$ bin/bkctl help
bkctl interacts and operates Apache BookKeeper clusters

Usage:  bkctl [flags] [command group] [commands]

Infrastructure commands :

    bookie          Commands on operating a single bookie
    bookies         Commands on operating a cluster of bookies
    cluster         Commands on administrating bookkeeper clusters

Ledger service commands :

    ledger          Commands on interacting with ledgers

Table service commands :

    namespace       Commands on operating namespaces
    table           Commands on interacting with tables
    tables          Commands on operating tables

Other commands :

    help            Display help information about it

---

In order to uphold a high standard for quality for code contributions, Apache BookKeeper runs various precommit
checks for pull requests. A pull request can only be merged when it passes precommit checks. However running all
the precommit checks can take a long time, some trivial changes don't need to run all the precommit checks. You
can check following list to skip the tests that don't need to run for your pull request. Leave them unchecked if
you are not sure, committers will help you:

• [skip bookkeeper-server bookie tests]: skip testing org.apache.bookkeeper.bookie in bookkeeper-server module.
• [skip bookkeeper-server client tests]: skip testing org.apache.bookkeeper.client in bookkeeper-server module.
• [skip bookkeeper-server replication tests]: skip testing org.apache.bookkeeper.replication in bookkeeper-server module.
• [skip bookkeeper-server tls tests]: skip testing org.apache.bookkeeper.tls in bookkeeper-server module.
• [skip bookkeeper-server remaining tests]: skip testing all other tests in bookkeeper-server module.
• [skip integration tests]: skip docker based integration tests. if you make java code changes, you shouldn't skip integration tests.
• [skip build java8]: skip build on java8. ONLY skip this when ONLY changing files under documentation under site.
• [skip build java9]: skip build on java9. ONLY skip this when ONLY changing files under documentation under site.

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Be sure to do all of the following to help us incorporate your contribution
quickly and easily:

If this PR is a BookKeeper Proposal (BP):

• Make sure the PR title is formatted like:
  <BP-#>: Description of bookkeeper proposal
e.g. BP-1: 64 bits ledger is support
• Attach the master issue link in the description of this PR.
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• Make sure the PR title is formatted like:
  <Issue #>: Description of pull request
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• Make sure tests pass via mvn clean apache-rat:check install spotbugs:check.
• Replace <Issue #> in the title with the actual Issue number.

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[sijie]

run pr validation
run integration tests
run bookkeeper remaining tests

[sijie]

run bookkeeper-server remaining tests

[sijie]

run bookkeeper-server replication tests