cli: Implement functions to manipulate namespaces

Unlike other runtimes, we have several components like qemu, kata-shim and
kata-proxy that should be placed in new namespace to improve the isolation and
security. The problem is that we need persistent namespaces and in the case of
the mount namespace (aka headache namespace ;)), it MUST BE created before
starting golang execution and in a new mount point with a propagation different
to shared, even worse, to make it persistent the new namespace
(/proc/PID/ns/mnt) MUST be 'bind mounted' by a process that doesn't know of the
existence of it. To get more information about this, see
github.com/karelzak/util-linux/commit/c84f2590dfdb8570beeb731e0105f8fe597443d1

With these new functions now is possible to join, create, remove and make
persistent namespaces. Next diagram can help us to understand how it works.

```
 ============================================================================
| Host's namespace                                    | New namespace!       |
|                                                     |                      |
| ---------------------------------------------------------------------------|
| C (no threads, no goroutines, NO GOLANG!)           |                      |
|                                                     |                      |
|  ---------   ---------------                        |                      |
| | Runtime |-|listen children|--|fork-setns-unshare|-|-------.              |
|  ---------   ---------------\                       |       |              |
|      |               |       \                      |       |              |
|    fork              |        Wait children &       |       |              |
|      |               |             exit             |       |              |
|----------------------------------------------------------------------------|
| Golang               |                              |       |              |
|      |        join,create,make,remove               |       |              |
|  ---------    persistent namespace                  |   ---------          |
| | Runtime |          |                              |  | Runtime |         |
|  ---------        --------                          |   ---------          |
|      |           |-Create |                         |       |              |
|  parse jsons     |-Start  |                         | Create,start,etc.    |
|  and cmdline     |-Delete |                         |   Containers         |
|       \          |-etc    |                         |       |              |
|        \         |--------                          |      exit            |
|         --subcmd-'                                  |                      |
|          \                                          |                      |
|           - exit                                    |                      |
 ============================================================================
```

fixes kata-containers#160

Depends-on: github.com/kata-containers/tests#946

Signed-off-by: Julio Montes <julio.montes@intel.com>