allow attaching to specific instance of service

fixes https://jira.zenoss.com/browse/ZEN-13288

DEMO1 - attach to specific instances of zope:
```
# plu@plu-9: serviced service status |grep Zope
    Zope		a8ki4roidmvqcyun1jkhu5weu				
      Zope_0		a8ki4roidmvqcyun1jkhu5weu_0	24s		plu-9	8da23a936996
      Zope_1		a8ki4roidmvqcyun1jkhu5weu_1	22s		plu-9	b026288579db

# plu@plu-9: serviced service attach zope
NAME	SERVICEID			INST	POOL		HOSTID		DOCKERID
Zope	a8ki4roidmvqcyun1jkhu5weu	1	default		570a276e	b026288579db0379491a6c5ee51e49c71e20242bf441c1c9639225dba3aaaab1
Zope	a8ki4roidmvqcyun1jkhu5weu	0	default		570a276e	8da23a93699605f318c7c3605a99c07b000799566c2d93ed933b438aaa0554dd
multiple results found; select one from list

# plu@plu-9: serviced service attach zope/0
[sudo] password for plu: 
bash-4.2# uname -n
8da23a936996
bash-4.2# exit
Connection to 172.17.42.1 closed.

# plu@plu-9: serviced service attach zope/1
[sudo] password for plu: 
bash-4.2# uname -n
b026288579db
bash-4.2# exit
Connection to 172.17.42.1 closed.

# plu@plu-9: serviced service attach default/zope/1
[sudo] password for plu: 
bash-4.2# uname -n
b026288579db
bash-4.2# exit
Connection to 172.17.42.1 closed.

```

DEMO2 - will attach remotely with ssh if current hostid != RunningService.HostID:
```
# plu@plu-9: serviced -v=1 service attach redis ps -wwef
I0801 09:30:30.291686 19498 nsenter.go:99] attach command for container:25d9d50a2abd70478f93c8b80086ba68d44b58d65962a329ba6c597dc25cd191 command: [/usr/bin/sudo /usr/bin/nsenter -m -u -i -n -p -t 24514 -- ps -wwef]
I0801 09:30:30.291941 19498 service.go:932] remote attaching with: [/usr/bin/ssh -t 172.17.42.1 -- /usr/bin/sudo /usr/bin/nsenter -m -u -i -n -p -t 24514 -- ps -wwef]
[sudo] password for plu: 
UID        PID  PPID  C STIME TTY          TIME CMD
root         1     0  0 12:51 ?        00:00:12 /serviced/serviced service proxy 1deukf4xjrnk6ojq3e0p7cw7l 0 /usr/sbin/redis-server /etc/redis.conf
root        13     1  0 12:51 ?        00:00:02 /usr/local/serviced/resources/logstash/logstash-forwarder -idle-flush-time=5s -old-files-hours=26280 -config /etc/logstash-forwarder.conf
root        19     1  0 12:51 ?        00:00:03 /usr/sbin/redis-server /etc/redis.conf
root     10313     0  0 13:49 ?        00:00:00 bash
root     17608     0  0 14:30 ?        00:00:00 ps -wwef
Connection to 172.17.42.1 closed.

# plu@plu-9: serviced -v=1 service attach redis 
I0801 09:30:35.134491 19640 nsenter.go:99] attach command for container:25d9d50a2abd70478f93c8b80086ba68d44b58d65962a329ba6c597dc25cd191 command: [/usr/bin/sudo /usr/bin/nsenter -m -u -i -n -p -t 24514 -- bash]
I0801 09:30:35.134712 19640 service.go:932] remote attaching with: [/usr/bin/ssh -t 172.17.42.1 -- /usr/bin/sudo /usr/bin/nsenter -m -u -i -n -p -t 24514 -- bash]
[sudo] password for plu: 
bash-4.2# exit
```