Supporting custom namespace for exec

[ahmetozer]

Exec function is previously re executes commands in same space of the container.
Now it supports to use host or provide different namespace from process id or namespace file.

ex:

sandal run --rm --name test -lw / --tmp 10 --env-all -- ifconfig
sandal exec --env-all --ns-net host test -- ifconfig

sandal run --rm --name test -lw / --tmp 10 --env-all -- ifconfig
ip netns create test2
sandal exec --env-all --ns-net file:/var/run/netns/test2 test --  ifconfig

sandal run --rm --name test -lw / --tmp 10 --env-all --  ifconfig
sandal exec --env-all --ns-net  pid:200 test -- ifconfig