Add validation for simple security checks

[robscott]

This involves 4 bits of configuration:

• security.runAsPriviliged: This container check should fail if container.securityContext.privileged is true
• security.runAsRootAllowed: This pod and container check should fail if pod.securityContext.runAsNonRoot or container.securityContext.runAsNonRoot is not true
• security.notReadOnlyRootFileSystem: This container check should fail if container.securityContext.readOnlyRootFileSystem is not true
• security.privilegeEscalationAllowed: This container check should fail if container.securityContext.allowPrivilegeEscalation is true

If I don't get to it first, this will also involve some changes to the existing config package Security struct.

[robscott]

Closed by #48