Mount config drive as loop device to supply data to cloud-init

Current configdrive approach is creating a partition on disk and write config data into it is iso format.
The downsides we've experienced are:
1. Waste disk space to accommodate small amount of data
2. Need a primary partition
     For images like coreos, which contains several primary partitions, config drive partition creation will fail due to limited primary partition number
3. For some qcow2 images, somehow the whole disk will be used up although the original image is small, leave no space for config drive partition.
4. See bug https://bugs.launchpad.net/ironic/+bug/1431527

The suggested new approach is:
1. Add an configurable option (configdrive_type) on the ironic side so that users can choose between loop device based config drive and partition based config drive.
2. On the ironic side, pass the option to ironic python agent in "prepare_image" method
3. If ironic python agent receives loop option, it shall do following steps after OS image is written to disk:
    a) partx -a $device, discover the new partitions
    b) vgchange -ay, discover new volumes
    c) check all disk and partition block devices, if the size is greater than 0, try mounting the disk/parition
    d) search for "fstab" file in the mounted disk/partition
    e) if "fstab" is found, make folder "/etc/cloud/configdrive" to save the config drive, and folder "/etc/cloud/cfgdrive_mnt" as mount point
    f) append an entry in "fstab" file
        /etc/cloud/configdrive/config.disk /etc/cloud/cfgdrive_mnt iso9660 loop 0 0

4. After reboot, cloud-init will detect the loop device and load data from it.