Fix heap_dump=tmp for jvmcfg (rebased onto dev_5_0)


This is the same as gh-3100 but rebased onto dev_5_0.

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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.