Generate manifest file

Generate manifest file

The manifest file stores important infomation such as image file name,
package format, compress format, partitions or disks. So when another program
want to parse image files, it can get the infomation more easily by reading
manifest file. There are two important kinds of format, see bellow:
loop:
   {
       "format": "loop",
       "version": "0.24",
       "loop": {
           "image_files": [
               "whole_rd_210-201403211800.tar.gz"
           ],
           "pack": ".tar.gz",
           "partitions": {
              "/opt": "data",
              "/opt/usr": "ums",
              "/": "platform"
          }
      },
      "created": "2014-03-21 18:02:07"
  }
raw:
   {
       "raw": {
          "image_files": [
               "ivi-201403211803-sda.raw.bz2",
               "ivi-201403211803-sdb.raw.bz2"
           ],
           "disks": [
               "sda",
               "sdb"
          ],
          "compress": "bz2",
          "sda": {
              "image": "ivi-201403211803-sda.raw.bz2",
              "bmap": "ivi-201403211803-sda.bmap"
          },
          "sdb": {
              "image": "ivi-201403211803-sdb.raw.bz2",
              "bmap": "ivi-201403211803-sdb.bmap"
          }
      },
      "format": "raw",
      "version": "0.24",
      "created": "2014-03-21 18:06:10"
  }
Change-Id: Ie9682bad79337319d1c1e6534b3cd4db25271124
Fixes: #1583

Conflicts:
	plugins/imager/fs_plugin.py
	plugins/imager/livecd_plugin.py
	plugins/imager/liveusb_plugin.py
	plugins/imager/loop_plugin.py
	plugins/imager/raw_plugin.py