Ticket #21: Targets are not correctly updated with '-n' option

If an empty nodeset is provided with the '-n' and '-x' options, the
target nodes were not correctly updated.
With '-n --only-nodes', if the nodeset was empty, milkcheck runs on all
targets specified in the configuration.
The '-x --exclude-nodes' option was not affected by this bug.

This patch fixes #21

Change-Id: I64bc3286a154b14a50f9815e14fb3448c8fe2896