This change moves the code specific to docker to kubelet/dockertools,
while leaving the common utility functions at its current package
(pkg/securitycontext).
When we deprecate dockertools in the future, the code will be moved to
pkg/kubelet/dockershim instead.
In podSecurityPolicy:
1. Rename .seLinuxContext to .seLinux
2. Rename .seLinux.type to .seLinux.rule
3. Rename .runAsUser.type to .runAsUser.rule
4. Rename .seLinux.SELinuxOptions
1,2,3 as suggested by thockin in #22159.
I added 3 for consistency with 2.