Executive: Permit OSSTEST_TASK=<refkey> (for static tasks)

Executive: Permit OSSTEST_TASK=<refkey> (for static tasks)

If OSSTEST_TASK is not set, we construct a <refkey> from the username
and the nodename, and look for a such a static task.  If OSSTEST_TASK
/is/ set would require it to contain `<taskid> <type> <refkey>'.

In this patch, permit OSSTEST_TASK to be set simply to the <refkey>.
This is much more convenient and doesn't involve manually looking up
taskids.  The risk of error seems negligible.

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>