fixes CC-51: replace docker import with tag with docker import and docke...

...r tag

DEMO:
```
root@ip-10-111-3-182:/opt/serviced# docker images
REPOSITORY          TAG                 IMAGE ID            CREATED             VIRTUAL SIZE
hello-world         latest              565a9d68a73f        6 weeks ago         922 B
root@ip-10-111-3-182:/opt/serviced# ./bin/serviced 
This master has been configured to be in pool: default
I0822 16:28:34.087649 18921 api.go:86] StartServer: [] (0)
I0822 16:28:34.111418 18921 manager.go:179] Checking isvcs container &{ContainerDescription:{Name:elasticsearch-logstash Repo:zenoss/serviced-isvcs Tag:v15 Command:/opt/elasticsearch-1.3.1/bin/elasticsearch Volumes:map[data:/opt/elasticsearch-1.3.1/data] Ports:[9100] HealthCheck:0x573290 Configuration:<nil> Notify:<nil> volumesDir:} client:0xc208068400 ops:0xc208004360}
I0822 16:28:34.113872 18921 manager.go:187] Looking for image tar /opt/serviced/images/zenoss/serviced-isvcs/v15.tar.gz
I0822 16:28:34.113986 18921 manager.go:136] Loading docker image zenoss/serviced-isvcs:v15 with docker import cmd: [docker -H unix:///var/run/docker.sock import -]
I0822 16:29:12.668853 18921 manager.go:145] Tagging imported docker image c51197638d01079567d795c36a8fe444541eddf1d848814e634aaa59e79cec9a with tag zenoss/serviced-isvcs:v15 using docker tag cmd: [docker tag c51197638d01079567d795c36a8fe444541eddf1d848814e634aaa59e79cec9a zenoss/serviced-isvcs:v15]
I0822 16:29:12.721756 18921 manager.go:179] Checking isvcs container &{ContainerDescription:{Name:zookeeper Repo:zenoss/serviced-isvcs Tag:v15 Command:/opt/zookeeper-3.4.5/bin/zkServer.sh start-foreground Volumes:map[data:/tmp] Ports:[2181 12181] HealthCheck:0x5724e0 Configuration:<nil> Notify:<nil> volumesDir:} client:0xc208068600 ops:0xc208004720}
...
```