Tested "find_resulce()" can find a backup by name or ID.
But some help messages and doc were "ID only", this patch
modify them.
Change-Id: I22dcc3a0b40f2bafabb6d26498158e2468037312
Cinder V2 API supports creating volume backup with "--force"
option. However, OSC doesn't support this argument. So this
patch add the "--force" option to allow users to back up a
in-use volume.
Change-Id: I326f8d6172b2830da4cf1317348af50142cc5490
Closes-Bug: #1596443