Support state_path in Zun

[hongbin]

Nova has the following:

  state_path = $pybasedir (StrOpt) Top-level directory for maintaining nova's state

The state_path is used for various purpose:

  instances_path = $state_path/instances
  networks_path = $state_path/networks
  quobyte_mount_point_base = $state_path/mnt
  buckets_path = $state_path/buckets
  glusterfs_mount_point_base = $state_path/mnt
  nfs_mount_point_base = $state_path/mnt
  scality_sofs_mount_point = $state_path/scality
  smbfs_mount_point_base = $state_path/mnt
  ca_path = $state_path/CA
  keys_path = $state_path/keys

It would be great if Zun has a state_path as well. Then, we can create various directory for storing container images, snapshot, and other data.