Fix data source keycloak_group with nested group

[chanhht]

This is the PR for fixing #333

The fix is not "complete", when there are multiple groups that match the filter name, it will return the first group it found (right now using DFS). This would cause some unexpected behaviors, for example, the data returned would change if new groups added with have the same with the data group name, or in case there are multiple nested groups share the same name, there is no option to get the desired one:

/core
--/developers
/cloud
--/developers

Unless we introduce another indicator (eg. path), I think this is the best we can try.
Notes: I am not too familiar with Golang if you found something weird, just let me know, thanks :)

[mrparkers]

thanks for the fix. this is probably the best we can do without some improvements to the Keycloak REST API.