Add coverage models

Add classes to manage the coverage values between the ERP and OTRS.

Creating the next classes:

• ADSLCoverage
• MMFibreCoverage
• VdfFibreCoverage

all these methods implement a method get_selector_values() as a class method to fill the fields in the wizard and the possible values of the new model CoverageInfo fields.

 classDiagram
      Coverage <-- ADSLCoverage
      Coverage <-- MMFibreCoverage
      Coverage <-- VdfFibreLCoverage
      Coverage : +get_selector_values() List
      ADSLCoverage : VALUES List
      MMFibreCoverage : VALUES List
      VdfFibreLCoverage : VALUES List