* If there is localrc exists, devstack will ignore local.conf.
Add some note to clarify this.
* Add VOLUME_BACKING_FILE_SIZE=20G when create cinder volume.
Since when create a default bay, it will require at least 5G * 2
volume, but the default cinder volume group size is only 10G, even
it will sitify 5G *2, but I got failed when use default volume group
size.
* And some other nit fixes.
Change-Id: I24c894d3799ee5aecde551a47c15b6a98546874a
ba6232ce3d