Improve mbed-tools --help text

[rwalton-arm]

Describe the bug
The help text for mbed-tools looks bad because Click truncates the short help text:

  configure  Generate an Mbed OS config CMake file and write it to a...
  deploy     Checks out Mbed program library dependencies at the revision...

Some of the information is also incorrect:

Options:
  --version        Display versions of all Mbed Tools packages.

We no longer have multiple mbed-tools packages so this text needs to change.

We also have a combination of imperative and indicative mood in the short help:

  import     Clone an Mbed project and library dependencies.
  new        Creates a new Mbed project at the specified path.

We should prefer imperative mood. So in the above example "Creates ..." should be "Create ...".

To Reproduce
Steps to reproduce the behavior:
mbed-tools --help

Expected behavior
Short help text is short enough so Click doesn't truncate it.
Help text information is correct and up-to-date with the current state of the tool.
Command descriptions use imperative mood.