Fixes and new remove_link_type_vlan function

Changes

• Updated developer guide to reflect Python 3 development and to not use pip
  from system repositories (as it is too old).

New

• Added function remove_link_type_vlan that allows to remove vlan
  virtual links created with add_link_type_vlan.

Fixes

• Fixed issue in add_link_type_vlan that ignored the ports mapping of the
  node.
• Fixed issue in add_link_type_vlan that registered the port mapping before
  creating it, causing potential issue if the creation failed.