allow empty device name when launching an instance

From comment 9 on Nova bug 1337821, the device name should be optional
and can cause issues if we force the user to select one and they enter
an incorrect value. We should make the field optional (and make sure
we still create the instance correctly in these cases.)

Change-Id: I7745b67bc52bbfbb98aa223b6e14ae2f96d81339
Closes-Bug: 1339760
Merge "allow empty device name when launching an instance"