implemented user-voicemail disassociation
replaced UserVoicemailNotExistsError by a AssociationNotExistsError, with a more explicit message
wrote scenarii for functionnal tests
don't repeat names and ids used in scenarios
use user_voicemail model instead of id
Merge branch '4689_dissociate_user_voicemail'
Merge branch 'master' of github.com:xivo-pbx/xivo-restapi