This prepares volume plugins to run things in containers instead of running
them on the host.
As consequence, a mount.Exec interface needs to be passed from VolumeHost
down to SioClient.
This commit introduces the following updates and fixes:
- Enable scaleIO volume multip-mapping based on accessMode
- No longer uses "default" as default values for storagepool & protection domain
- validates capacity when capacity is zero
- Better naming for PV and volume
- make mount ro when accessModes contains ROM
yameiwang