Improve group arguments allowing to specify input type (id or name)

In add, joingroup, leavegroup subcommands, group can be passed by id, by name
or by id_or_name as a positional argument.
Fix inverted id and name in group addition message

Also explicitely state name and id in user addition message
Refactor find_group() method and expose group ID/name ambiguity

find_group() now systematically looks up for group by ID and by name for a
given input. Found groups are resolved in a second step and a warning/error is
raised is multiple groups are found, e.g. for two different group where
id_or_name is the ID of the one and the name of the other.
Do not return group if ambiguous group is found in find_group()
Merge pull request #1022 from sbesson/10677_integergroupname

CLI: Improve group arguments allowing to specify input type (see #10677)