Modify _ApplyContainerMods function to lookup NICs/Disks not only by their
index inside the container, but also by their UUID or name. Abstract the
lookup code in new GetItemFromContainer function.

Make type of identifier in "opcode._TestInstSetParamsModList" to be
TInt or TString.

Modify gnt-instance client to not check if identifier is an integer.

Signed-off-by: Christos Stavrakakis <cstavr@grnet.gr>
Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>
Reviewed-by: Helga Velroyen <helgav@google.com>