BUG-972: correct Precondition

This is just a tiny adjustment of a Precondition, which should have been
triggered before this access. isPresent() returns true/false, which will
be autoboxed, so will always pass checkNotNull. use checkArgument()
instead, which will ensure that isPresent() == true.

Change-Id: I53fe6913c7a0f8073a193bc76c0792059c26fe2a
Signed-off-by: Robert Varga <rovarga@cisco.com>
Bug 809: Enhancements to the toaster example

See https://bugs.opendaylight.org/show_bug.cgi?id=809 for details.

Need someone to commit this!

Change-Id: Icb9697de77d844d816ec61a0e7a6ac2100d185ae
Signed-off-by: tpantelis <tpanteli@brocade.com>