Allow downstream impl of mbed dm to print help

[theotherjimmy]

The mbed dm subcommand has a complicated invocation. Instead of
duplicated the argument parser, I elected to pass the implementation
of mbed dm --help to the "downsteam implementation" (the python
script in mbed-os).

Resolves help issues in #747

[screamerbg]

Single line implementation! Love it! 👍

[screamerbg]

@MarceloSalazar Please try and review.

[MarceloSalazar]

Thanks for looking into this. Now there is some help :)

However, as user I'm confused on which command I need to run.
See output:

C:\mbed>mbed dm -h
[mbed] Auto-installing missing Python modules...
usage: device_management.py [-h] [-l {debug,info,warning,exception}]
                            [--version]
                            {create,parse,verify,cert,init,sign,update} ...

Create or transform a manifest. Use C:\mbed\mbed-
os\tools\device_management.py [command] -h for help on each command.

positional arguments:
  {create,parse,verify,cert,init,sign,update}
    create              Create a new manifest
    parse               Parse an existing manifest
    verify              Verify an existing manifest
    cert                Create or examine a certificate
    init                Set default values for manifests
    sign                Sign an existing manifest
    update              Work with the Mbed Cloud Update service

optional arguments:
  -h, --help            show this help message and exit
  -l {debug,info,warning,exception}, --log-level {debug,info,warning,exception}
  --version             display the version

Why do I need to care about "device_management.py" ?
I'm not interested in the internals of Mbed OS tools.

My interface is (and should continue to be) Mbed CLI

[theotherjimmy]

@MarceloSalazar That's something that should be changed in device_management.py.

[theotherjimmy]

@MarceloSalazar I'll create a PR to have the device_managment.py help print correctly. My prototype does this:

$  mbed dm -h
[mbed] Auto-installing missing Python modules...
---
usage: mbed device-management [-h] [-l {debug,info,warning,exception}]
                              [--version]
                              {create,parse,verify,cert,init,sign,update} ...

Create or transform a manifest. Use mbed device-management [command] -h for
help on each command.

positional arguments:
  {create,parse,verify,cert,init,sign,update}
    create              Create a new manifest
    parse               Parse an existing manifest
    verify              Verify an existing manifest
    cert                Create or examine a certificate
    init                Set default values for manifests
    sign                Sign an existing manifest
    update              Work with the Mbed Cloud Update service

optional arguments:
  -h, --help            show this help message and exit
  -l {debug,info,warning,exception}, --log-level {debug,info,warning,exception}
  --version             display the version

[theotherjimmy]

@MarceloSalazar Mbed OS PR up: ARMmbed/mbed-os#8104