Unify semantics for group ownership in OMERO.py integration test

Utilities for creating new users and/or clients in the library were using an
admin keyword as a way to set the group ownership. This commit aligns all the
integration tests to use the current semantics.
Review integration tests for sudo functionality

Combine sudo tests into a single parametrized test in multi-group mode testing
all use cases
Prevent confusion between admin service and admin user
Force new connection every time
Check group owner cannot sudo in another group
Convert check_sudoer into an local method
Add test for context change while sudo
Merge pull request #41 from sbesson/group-sudo

Integration tests for group sudo