allow attaching to specific instance of service

DEMO:
```
# plu@plu-9: serviced service attach zope
NAME	SERVICEID			INSTANCE	DOCKERID
Zope	ezo0suqvfunwt96pc00eucodn	1		1a6a536cb80752b867382459a6b05ae65b3353b916521725e3888a2c01711912
Zope	ezo0suqvfunwt96pc00eucodn	0		535c199a670165fbc7e4cec363a86708ed662d855e09c3a92b808a48c9cb974b
multiple results found; select one from list

# plu@plu-9: serviced service attach zope:0
/
# root@535c199a6701: exit

# plu@plu-9: serviced service attach zope:1
/
# root@1a6a536cb807: exit
```