In magnum server side, docker_volume_size allowed minimum value is 1GB.
In fact, the minimum value should be 3GB when docker-storage-driver is
devicemapper, because the limitation of docker-storage-setup. So, I add
check on "--docker-volume-size". If "--docker-storage-driver=devicemapper",
the minimum docker-volume-size should be 3GB, otherwise it should 1GB.
Change-Id: I53c6a9c93ee156fa9afabdbd72a87bb4044d71ca
Closes-Bug: #1583878