Correctly parse build URL and FILE (fix failing virtualbox builds)

Since the support for primitive result sets with the xpath query parameter in
the Jenkins REST API has been dropped, text() cannot be used anymore. Instead
we parse the XML block read from the API for the latest successful build URL
and server file.
Allow OMERO_BUILD to be set by environment variable
Use OMERO_JOB rather than OMERO_BUILD to pass job name
Allow to set and pass OMERO_JOB envvar from top-level omerovm.sh
Merge pull request #913 from sbesson/virtualbox_fix

Correctly parse build URL and FILE (fix failing virtualbox builds)