Try to fix 'omename' not found in robotframework webadmin

Several times we've seen robotframework test fail with:
```ValueError: Element locator 'omename' did not match any elements.```

http://ci.openmicroscopy.org/view/Failing/job/OMERO-5.0-merge-robotframework/144/console
http://ci.openmicroscopy.org/view/Failing/job/OMERO-5.0-merge-robotframework/155/console

Not sure if the selector by name is too slow or fragile for some reason, but it may be more robust to specify ID or to explicitly use xpath to state that we are selecting by name.

If this seems to work, we can use the same logic in other selectors on this page (more commits to this PR).