lxd/storage/drivers/volume: Differentiates between volume config and pool config
lxd/storage/drivers/volume: Differentiates between volume config and pool config

- Stores pool config inside Volume.
- Allows external access to volume config via Config() function.
- Allows access to "expanded" config keys using ExpandedConfig(key) function which first checks if key exists in Volume config and if not looks for "volume.{key}" in pool's config.

Signed-off-by: Thomas Parrott <6b778ce645fb0e3dde76d79eccad490955b1ae74@canonical.com>

mjeanson/lxd,hallyn/lxd,atxwebs/lxd,Skarlso/lxd,stgraber/lxd,atxwebs/lxd,Skarlso/lxd,stgraber/lxd,Skarlso/lxd,lxc/lxd,hallyn/lxd,hallyn/lxd,Skarlso/lxd,lxc/lxd,stgraber/lxd,stgraber/lxd,hallyn/lxd,atxwebs/lxd,stgraber/lxd,hallyn/lxd,mjeanson/lxd,lxc/lxd,stgraber/lxd,lxc/lxd,stgraber/lxd,lxc/lxd,mjeanson/lxd,atxwebs/lxd,hallyn/lxd,lxc/lxd,lxc/lxd