remove nsenter and its dependence on sudo

DEMO:
```
# plu@plu-9: serviced service start /redis
Service scheduled to start.
~/src/europa/src/golang/src/github.com/control-center/serviced
# plu@plu-9: serviced service attach /redis/0
bash-4.2# exit
~/src/europa/src/golang/src/github.com/control-center/serviced
# plu@plu-9: serviced -v=1 service attach /redis/0
I1022 10:32:52.190434 09487 docker_exec.go:71] attach command for container:b7125fdfb5271ebee69b1929ae23144e3c2b628aee21a501561edecdf558c2d9 command: [/usr/bin/docker exec -i -t b7125fdfb5271ebee69b1929ae23144e3c2b628aee21a501561edecdf558c2d9 /bin/bash]
I1022 10:32:52.190690 09487 docker_exec.go:32] exec command for container:b7125fdfb5271ebee69b1929ae23144e3c2b628aee21a501561edecdf558c2d9 command: [/usr/bin/docker exec -i -t b7125fdfb5271ebee69b1929ae23144e3c2b628aee21a501561edecdf558c2d9 /bin/bash]
bash-4.2# 
```