Fix heap_dump=tmp for jvmcfg

Use of `bin/omero config set omero.jvmcfg.heap_dump tmp`
was broken shortly before release and apparently never
worked. Rather than change the return value of get_heap_dump
to a list, the `tmp` shortcut now does the same that a user
could do, namely includes the HeapDumpPath argument in the
append section, though possibly with easier white-space
handling than the user could perform themselves.

Reported by Seb Simard
http://lists.openmicroscopy.org.uk/pipermail/ome-devel/2014-October/003021.html

/cc @zeb Note: candidate for rebase.
The whole element turns into a single `<option>...</option>` element. From `etc/grid/generated.xml`:
```
 <option>-XX:HeapDumpPath=/tmp</option>
```
so it should work out fine.