Make PubSubManager.getNode(String) more robust

I've got reports from users that in some cases there can be multiple
identities. Not just one, and in this case, the node type may not be the
first identity. We now iterate over all identities until we either found
one of type "leaf" or "collection".

For example one user reports an ejabberd with PEP case, where the first
identity is of type "pep", the second of type "leaf" and a third one
with category "account" and type "registered".

Also extend DiscoverInfo API with hasIdentity(String, String) and
getIdentities(String, String).