Revert "metadata: don't fail if no network config was provided"

This reverts commit c1f373e648e1d8950faef668b5a70968db2408a3.
configdrive: return an empty network config when filename is empty

Additionally, don't bother checking for a network config if it isn't going to
be processed.
Merge pull request #233 from crawford/configdrive

configdrive: don't fail if no network config was provided