Update doc volumes.md to make sure mount point is created before mounting.
Merge pull request #3169 from dchen1107/pd

Update doc volumes.md to make sure mount point is created before mountin...
Util to test if all ptr fields are nil
Add a util/mount pkg
Move pkg/volume under kubelet
Add volume plugins scaffolding
Disable volume test while plugin-izing
empty_dir volume plugin
Move kubelet-specific code to kubelet
host_mount volume plugin
git_repo volume plugin
gce_pd volume plugin
Remove non-plugin mode for volumes
Use mount.Interface for GCE
Add a new gce_pd_test
Move plugins to their own pkgs
Hand merge #2886
WIP Clean up tests