allow attaching to specific instance of service

fixes https://jira.zenoss.com/browse/ZEN-13288

DEMO - attach to specific instances of zope either remotely or locally:
```
zenny@ip-10-111-3-197:~$ serviced service status | grep Zope
    Zope		1li6e345hn8q7x85llf4njyyf					
      Zope_0		1li6e345hn8q7x85llf4njyyf_0	3m38s	ip-10-111-3-197		6645e0549d92
      Zope_1		1li6e345hn8q7x85llf4njyyf_1	4m37s	ip-10-111-3-75		68d4062b9261
      Zope_2		1li6e345hn8q7x85llf4njyyf_2	4m29s	ip-10-111-3-197		9ee03d1c35ec
      Zope_3		1li6e345hn8q7x85llf4njyyf_3	4m48s	ip-10-111-3-75		da7aefed715a
      Zope_4		1li6e345hn8q7x85llf4njyyf_4	4m13s	ip-10-111-3-197		5fb53e4d8b99
      Zope_5		1li6e345hn8q7x85llf4njyyf_5	4m25s	ip-10-111-3-75		6e4c9408702c

zenny@ip-10-111-3-197:~$ serviced service attach zope | sort -k +3
multiple results found; specify unique item from list
Zope	1li6e345hn8q7x85llf4njyyf	0	default		ip-10-111-3-197		10.111.3.197	6645e0549d92
Zope	1li6e345hn8q7x85llf4njyyf	1	default		ip-10-111-3-75		10.111.3.75	68d4062b9261
Zope	1li6e345hn8q7x85llf4njyyf	2	default		ip-10-111-3-197		10.111.3.197	9ee03d1c35ec
Zope	1li6e345hn8q7x85llf4njyyf	3	default		ip-10-111-3-75		10.111.3.75	da7aefed715a
Zope	1li6e345hn8q7x85llf4njyyf	4	default		ip-10-111-3-197		10.111.3.197	5fb53e4d8b99
Zope	1li6e345hn8q7x85llf4njyyf	5	default		ip-10-111-3-75		10.111.3.75	6e4c9408702c
NAME	SERVICEID			INST	POOL		HOST			HOSTIP		DOCKERID

zenny@ip-10-111-3-197:~$ serviced -v=1 service attach zope/0
I0806 01:58:07.761722 23934 nsenter.go:99] attach command for container:6645e0549d922db3a9e55ba38e88f679f68edea21a6b9eaa31e0e19de16dd4f8 command: [/usr/bin/sudo /usr/bin/nsenter -m -u -i -n -p -t 32061 -- /bin/bash]
I0806 01:58:07.761866 23934 nsenter.go:57] exec command for container:6645e0549d922db3a9e55ba38e88f679f68edea21a6b9eaa31e0e19de16dd4f8 command: [/usr/bin/sudo /usr/bin/nsenter -m -u -i -n -p -t 32061 -- /bin/bash]
bash-4.2# uname -n
6645e0549d92
bash-4.2# exit
exit

zenny@ip-10-111-3-197:~$ serviced -v=1 service attach zope/1
I0806 01:58:23.648768 27824 service.go:948] remote attaching with: [/usr/bin/ssh -t 10.111.3.75 -- serviced --endpoint 10.111.3.197:4979 service attach zope/1]
bash-4.2# uname -n
68d4062b9261
bash-4.2# exit
exit
Connection to 10.111.3.75 closed.

zenny@ip-10-111-3-197:~$ serviced -v=1 service attach zope/0 ps -wwef
I0806 01:58:37.024483 31019 nsenter.go:99] attach command for container:6645e0549d922db3a9e55ba38e88f679f68edea21a6b9eaa31e0e19de16dd4f8 command: [/usr/bin/sudo /usr/bin/nsenter -m -u -i -n -p -t 32061 -- ps -wwef]
I0806 01:58:37.026913 31019 nsenter.go:57] exec command for container:6645e0549d922db3a9e55ba38e88f679f68edea21a6b9eaa31e0e19de16dd4f8 command: [/usr/bin/sudo /usr/bin/nsenter -m -u -i -n -p -t 32061 -- ps -wwef]
UID        PID  PPID  C STIME TTY          TIME CMD
root         1     0  4 02:54 ?        00:00:11 /serviced/serviced service proxy 1li6e345hn8q7x85llf4njyyf 0 su - zenoss -c "/opt/zenoss/bin/runzope" 
root        18     1  0 02:54 ?        00:00:00 /usr/local/serviced/resources/logstash/logstash-forwarder -idle-flush-time=5s -old-files-hours=26280 -config /etc/logstash-forwarder.conf
root        79     1  0 02:54 ?        00:00:00 su - zenoss -c /opt/zenoss/bin/runzope
zenoss      80    79 10 02:54 ?        00:00:26 python2.7 /opt/zenoss/zopehome/runzope -C /opt/zenoss/etc/zope.conf
root      3607     1  0 02:58 ?        00:00:00 sh -c /tmp/answering806575903
root      3608  3607  0 02:58 ?        00:00:00 curl -o /dev/null -w %{redirect_url} -s http://localhost:9080/zport/dmd
root      3609  3607  0 02:58 ?        00:00:00 grep -q acl_users
root      3611     0  0 02:58 ?        00:00:00 ps -wwef

zenny@ip-10-111-3-197:~$ serviced -v=1 service attach zope/1 ps -wwef
I0806 01:58:46.428103 00906 service.go:948] remote attaching with: [/usr/bin/ssh -t 10.111.3.75 -- serviced --endpoint 10.111.3.197:4979 service attach zope/1 ps -wwef]
UID        PID  PPID  C STIME TTY          TIME CMD
root         1     0  3 02:53 ?        00:00:11 /serviced/serviced service proxy 1li6e345hn8q7x85llf4njyyf 1 su - zenoss -c "/opt/zenoss/bin/runzope" 
root        17     1  0 02:53 ?        00:00:00 /usr/local/serviced/resources/logstash/logstash-forwarder -idle-flush-time=5s -old-files-hours=26280 -config /etc/logstash-forwarder.conf
root        51     1  0 02:53 ?        00:00:00 su - zenoss -c /opt/zenoss/bin/runzope
zenoss      52    51  9 02:53 ?        00:00:30 python2.7 /opt/zenoss/zopehome/runzope -C /opt/zenoss/etc/zope.conf
root      5013     1  0 02:58 ?        00:00:00 sh -c /tmp/answering937634037
root      5016  5013  0 02:58 ?        00:00:00 curl -o /dev/null -w %{redirect_url} -s http://localhost:9080/zport/dmd
root      5017  5013  0 02:58 ?        00:00:00 grep -q acl_users
root      5019     0  0 02:58 ?        00:00:00 ps -wwef
Connection to 10.111.3.75 closed.
zenny@ip-10-111-3-197:~$ 
```