srm: simplify handling of time unit in srm.xml

    replaces TimeUnit.valuOf('${xxx.yyy.unit}'} with just '${xxx.yyy.unit}'

    Ticket:
    Patch: http://rb.dcache.org/r/5808/
    Acked-by: Gerd Behrmann
    Target: trunk
    Require-book: no
    Require-notes: no