remove nsenter and its dependence on sudo

DEMO:
```
# plu@plu-9: serviced service start /redis
Service scheduled to start.

# plu@plu-9: serviced service attach /redis/0 
bash-4.2# exit

# plu@plu-9: serviced -v=1 service attach /redis/0
I1023 15:38:28.288281 22991 docker_exec.go:77] attach command for container:d79327dc9724ca32af938d6eb251fed000a1f2acf1ee46c8306b03daa143c930 command: [/usr/bin/docker exec -t -i d79327dc9724ca32af938d6eb251fed000a1f2acf1ee46c8306b03daa143c930 /bin/bash]
I1023 15:38:28.288517 22991 docker_exec.go:32] exec command for container:d79327dc9724ca32af938d6eb251fed000a1f2acf1ee46c8306b03daa143c930 command: [/usr/bin/docker exec -t -i d79327dc9724ca32af938d6eb251fed000a1f2acf1ee46c8306b03daa143c930 /bin/bash]
bash-4.2# exit

# plu@plu-9: serviced service attach /redis/0 ps -wwef
UID        PID  PPID  C STIME TTY          TIME CMD
root         1     0  0 20:34 ?        00:00:00 /serviced/serviced service proxy 3p35aqh3pg78s4aqp7bbpvpwj 0 /usr/bin/redis-server /etc/redis.conf
root        15     1  2 20:34 ?        00:00:00 /usr/local/serviced/resources/logstash/logstash-forwarder -idle-flush-time=5s -old-files-hours=26280 -config /etc/logstash-forwarder.conf
root        19     1  0 20:34 ?        00:00:00 /usr/bin/redis-server *:6379
root        29     0  0 20:34 ?        00:00:00 ps -wwef

# plu@plu-9: serviced service attach /redis/0 ps -wwef | tee /dev/null
UID        PID  PPID  C STIME TTY          TIME CMD
root         1     0  0 20:34 ?        00:00:00 /serviced/serviced service proxy 3p35aqh3pg78s4aqp7bbpvpwj 0 /usr/bin/redis-server /etc/redis.conf
root        15     1  1 20:34 ?        00:00:00 /usr/local/serviced/resources/logstash/logstash-forwarder -idle-flush-time=5s -old-files-hours=26280 -config /etc/logstash-forwarder.conf
root        19     1  0 20:34 ?        00:00:00 /usr/bin/redis-server *:6379
root        45     0  0 20:35 ?        00:00:00 ps -wwef

# plu@plu-9: echo hi | serviced service attach /redis/0 ps -wwef 
UID        PID  PPID  C STIME TTY          TIME CMD
root         1     0  0 20:34 ?        00:00:00 /serviced/serviced service proxy 3p35aqh3pg78s4aqp7bbpvpwj 0 /usr/bin/redis-server /etc/redis.conf
root        15     1  0 20:34 ?        00:00:00 /usr/local/serviced/resources/logstash/logstash-forwarder -idle-flush-time=5s -old-files-hours=26280 -config /etc/logstash-forwarder.conf
root        19     1  0 20:34 ?        00:00:00 /usr/bin/redis-server *:6379
root        51     0  0 20:35 ?        00:00:00 ps -wwef
```