Issue #435: Add the possibility to use the same connector with different connector variables

[CherfaElyes]

• Added the possibility to define additional connectors, which are connectors with variables where the user can customize variables values.
• It is possible to define many instances of the same connector, and this by customizing values for each instance.
• Updated unit tests to make sure everything is working.
• Tested the changes on MetricsHub.
• Updated the MetricsHub documentation to explain how to define and customize connector variables.
• Updated MetricsHub maven plugin to generate additionalConnectors configuration.

Tests

Two additional connectors (same connector twice)

In this case, connectors compiled filename will take as value the connectorId.

One additional connector without uses

No additional connectors configured, only connector with variables is forced.

[iguitton]

@CherfaElyes , @NassimBtk: I reviewed and updated the user documentation. If you are fine with my version, you can approve and merge this PR. Thanks.

[CherfaElyes]

@CherfaElyes , @NassimBtk: I reviewed and updated the user documentation. If you are fine with my version, you can approve and merge this PR. Thanks.

Thank you @iguitton for the review!