mbed dm: Remove references to internal script from help

[theotherjimmy]

Description

Now that we are delegating the help text to device_managment.py,
it should print out help as if it's invoked with mbed device-management.
This PR changes the argument parser and the help text to do just that.

Pull request type

[x] Fix
[ ] Refactor
[ ] Target update
[ ] Functionality change
[ ] Breaking change

[theotherjimmy]

When combined with ARMmbed/mbed-cli#751:

$  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

[MarceloSalazar]

This is great! Thank you!

[cmonr]

/morph build

[cmonr]

Thanks for the sample output!

[mbed-ci]

Build : SUCCESS

Build number : 3063
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/8104/

Triggering tests

/morph test
/morph export-build
/morph mbed2-build

[mbed-ci]

Exporter Build : SUCCESS

Build number : 2677
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/8104/

[cmonr]

/morph test

[mbed-ci]

Test : SUCCESS

Build number : 2890
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/8104/2890