Start CT in a new user namespace: 1:1 user mapping

Now CT starts in a new user namespace. This allows us:

• to remove our capabilities (CAP_VE_*)
• to improve security of our containers, because a process doesn't have privileges outside the container

Here is a good article about user namespaces https://lwn.net/Articles/532593/

Users should not notice these changes, everything should work as before.

Testing

• need to execute tests to check security of containers
• execute all tests, because these changes are touching very general parts