Add server, health and config sub-cmd to regproxy cmd

[fisuda]

Proposed changes

This PR adds server, health and config sub-command to regproxy command.
It changes a command syntax in regproxy command.

• Before

ngsi --stderr info regproxy --host orion-with-keyrock

• After

ngsi --stderr info regproxy server --host orion-with-keyrock

Types of changes

What types of changes does your code introduce to the project: Put an x in the boxes that apply

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Update only documentation, not any source code.

Checklist

Put an x in the boxes that apply. You can also fill these out after creating the PR. If you're unsure about any of
them, don't hesitate to ask. We're here to help! This is simply a reminder of what we are going to look for before
merging your code.

• I have read the CONTRIBUTING doc
• I have signed the CLA
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)
• Any dependent changes have been merged and published in downstream modules

Further comments

N/A

[coveralls]

Coverage remained the same at 100.0% when pulling 56a06e9 on fisuda:improve/regproxy into f5256a8 on lets-fiware:main.