Add security context support by creating a user accessible directory beforehand

The permission of directory is 0770, because when a user login as group
e.g. group 1000, they're also belong to group 0. The default permission
for host path is 0700, which prevent group 0 from accessing the directory.

Fixes https://github.com/rancher/local-path-provisioner/issues/7 .

Also added test case `security-context.yaml` derived from @unguiculus 's
comment on the issue.