slapos_cloud: remove constraints on software instance certificate, revoke previous instance certificate when request a new one.

If a valid certificate exists and Node try to request a new certificate, the old certificate is revoked first.

slapos cli: register computer uses new certificate generation style, configure client generate csr with random CN

Store Certificate ID into a new portal type, update getcertificate and revokeCertificate for person, computer, software instance

-  Move all Computer attribute initialization into Computer constructor
-  Rename 'construct' to 'format' because we are in slap.format
-  Introduce Manager base class
-  Implement CGroupManager to take care of cgroup's cpuset

-  Add TUN class
-  Check Interface is a bridge before performing bridgey operations
-  Replace outdated 'tunctl' binary with 'ip tunctl'

  This raise my lead to a significant change on the API, which
  may require to refactor several tests, to this change keep s a
  backward compatible API.