Fix InstanceAlreadyExistsException in Orchard

[jthywiss]

Try Orc is repeatedly (several times per week on orc.csres) throwing:

javax.management.InstanceAlreadyExistsException: orc.orchard:type=GuestOnlyAccounts,name=""
java.lang.AssertionError: javax.management.InstanceAlreadyExistsException: orc.orchard:type=GuestOnlyAccounts,name=""
	at orc.orchard.jmx.JMXUtilities.registerMBean(JMXUtilities.java:54)
	at orc.orchard.Accounts.<init>(Accounts.java:74)
	at orc.orchard.GuestOnlyAccounts.<init>(Accounts.java:311)
	at orc.orchard.Accounts.<clinit>(Accounts.java:43)
	at orc.orchard.AbstractExecutorService.getAccounts(AbstractExecutorService.java:47)
	at orc.orchard.AbstractExecutorService.submit(AbstractExecutorService.java:78)
	at orc.orchard.soap.ExecutorService.submit(ExecutorService.java:108)
	at orc.orchard.AbstractExecutorService.compileAndSubmit(AbstractExecutorService.java:91)
	at orc.orchard.soap.ExecutorService.compileAndSubmit(ExecutorService.java:102)
	......