rest user/group drivers: refactor and fix user type for lw accounts

[gmgigi96]

This PR add the following fix/enhancement to the rest user and group drivers:

• Fix the user type for lightweight accounts, using the source field to differentiate between a primary and lw account
• Remove all the code with manual parsing of the json returned by the CERN provider
• Introduce pagination for GetMembers method in the group driver
  • the result without pagination was truncated to the first 1000 items
• Reduced network transfer size by requesting only needed fields for GetMembers method