Volume size could be specified to create volume

1. Provide a "volume_size" config at VolumeGroup
2. Make size as optional prameter, modify all volumes_client to
read the size from CONf if size is None.
3. Modify the testcase: test_volume_create_get_update
_delete_from_image to call create_volume with no size
parameter specified.

Change-Id: I2897f6d6bd970f73867f56d9d23a768cafcbfd80
Closes-Bug: #1293885
7 files changed