Robot test fixes (rebased onto dev_5_0)


This is the same as gh-3126 but rebased onto dev_5_0.

----

This fixes a couple of failing tests from http://ci.openmicroscopy.org/job/OMERO-5.1-merge-robotframework/47/console
They were failing due to changes in the webclient code.

Tests that are fixed are:
```
Web.Webadmin Create Group And User :: A test suite with a single test for v...
==============================================================================
Create Group :: Tests group creation                                  | FAIL |
ValueError: Element locator 'xpath=//ul[@class='chzn-choices']' did not match any elements.
```
and 
```
Web.Webadmin Login :: A test suite with a single test for valid login.\n\nT...
==============================================================================
Login Page :: Tests elements on the login page                        | FAIL |
Page should have contained image 'xpath=//div[@id='login-logo']/img' but did not
```

Other failing tests I couldn't repeat locally on Chrome or Firefox. They may have been caused by slow loading on root login (which should be fixed by https://github.com/openmicroscopy/openmicroscopy/pull/3123 or on Javascript failure in Chrome (although there is no error message and all we see in the failure to grab a screenshot when the tests fail).