utils: Add nfs prevalication

Abstract a helper function to parse cmd result.
Usage:
    (1)get cmd result with subprocess.call or subprocess.Popen
    (2) call pass_cmd_output to get formated outputs
Example:
    blkid = subprocess.Popen(["cat", "/proc/mounts"],
                stdout=subprocess.PIPE, stderr=subprocess.PIPE)
    outs = blkid.communicate()[0]
    output_items= ['path', 'mnt_point', 'type', 'option']
        utils.parse_cmd_output(outs, output_items)
Sample output:
    [{'path': '/dev/sda8', 'type': 'ext4',
      'option': 'rw,relatime,data=ordered', 'mnt_point': '/home'},
     {'path': 'localhost:/home/royce/isorepo', 'type': 'nfs4',
      'option': 'rw...addr=127.0.0.1', 'mnt_point': '/mnt'}]

    To prevent future mount of nfs path will hang,
    we will try nfs path with a quick mount,
    if this check fails, report warning to user.

Signed-off-by: Royce Lv <lvroyce@linux.vnet.ibm.com>
Integrate nfs path check before create nfs pool

Before creating nfs pool, reload the prevalidation for the path
and see if it is able to complish a quick mount.
If not, deny this storage pool from being created.

Signed-off-by: Royce Lv <lvroyce@linux.vnet.ibm.com>