Fix for DTACLOUD-437 Empty user-data genrated on rhevm3.1 instance

User data file was not being passed correctly to rbovirt.

Payload was incorrectly structured using "_file" which should be "file".
The suspicion is a Nokogiri change likely surfaced this as it used to
work fine with "_file".


Signed-off-by: Joe VLcek <jvlcek@redhat.com>