Fix leapp prints nothing when running with no args on python3.6+

[MichalHe]

A change introduced in the stdlib argparse module accidentally caused
all subparsers to become optional instead of required as in python 2.7.
This change therefore causes leapp to print nothing and exit with error
code 0 when being executed without any arguments.

This commit marks the subparser action as required, so that the
python3.6 module recognizes this situation and prints the usage message.

To provide a meaningful message on python3.6 the subparser action was
edited to use 'command' metavar and the 'title' option was dropped.

Tasks to be done:

• verify that the usage message when using python2.7 did not become somehow unreadable due to the metavar/title changes performed.

[centos-ci]

Can one of the admins verify this patch?

[github-actions]

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[leapp-bot]

This PR has been linked in issue tracker (#OAMG-5488).

[MichalHe]

The usage message running python2.7 without any arguments is changed by this PR to (notice the added metavar command):

usage: leapp [-h] [--version] command ...

Optional arguments:
  -h, --help  show this help message and exit
  --version   show program's version number and exit

Required arguments:
  command
    answer    Manage answerfile generation: register persistent user choices
              for specific dialog sections
    upgrade   Upgrade the current system to the next available major version.
    list-runs
              List previous Leapp upgrade executions
    preupgrade
              Generate preupgrade report
    rerun     Re-runs the upgrade from the given phase and using the
              information and progress from the last invocation of leapp
              upgrade.
error: too few arguments

Running leapp with no arguments on python3.6 outputs (with exit code 2):

usage: leapp [-h] [--version] command ...

Optional arguments:
  -h, --help  show this help message and exit
  --version   show program's version number and exit

Required arguments:
  command
    answer    Manage answerfile generation: register persistent user choices
              for specific dialog sections
    list-runs
              List previous Leapp upgrade executions
    preupgrade
              Generate preupgrade report
    rerun     Re-runs the upgrade from the given phase and using the
              information and progress from the last invocation of leapp
              upgrade.
    upgrade   Upgrade the current system to the next available major version.
error: the following arguments are required: command

[pirat89]

works as expected.