Count (patch bomb :) )

Hi,

In this PR, i'm:
-reworking deleteUser, deleteGroup
-moving groupPath from auth to conf
-introducing getUsersCountFromConf, userExistsInConf, listUsersFromConf and switch to them
-clean up sql schema (put good primary key, ...)

Work to do (in the future):
-change getAuthScheme
-Add db transactions
-...