replace deprecated base:latest with ubuntu:latest

docker 1.3.3 and later no longer support base:latest image.

ISSUE:
```
# plu@plu-9: docker version
Client version: 1.3.3
Client API version: 1.15
Go version (client): go1.3.3
Git commit (client): d344625
OS/Arch (client): linux/amd64
Server version: 1.3.3
Server API version: 1.15
Go version (server): go1.3.3
Git commit (server): d344625

~/src/europa/src/golang/src/github.com/control-center/serviced/commons/docker
# plu@plu-9: go test -v
=== RUN TestContainerCommit
--- FAIL: TestContainerCommit (0.00 seconds)
	container_test.go:39: can't create container:  API error (500): image ID '27cf784147099545' is invalid
		
=== RUN TestOnContainerStart
--- FAIL: TestOnContainerStart (0.00 seconds)
	container_test.go:84: can't create container:  API error (500): image ID '27cf784147099545' is invalid
```

DEMO:
```
~/src/europa/src/golang/src/github.com/control-center/serviced/commons/docker
# plu@plu-9: go test -v
=== RUN TestContainerCommit
--- PASS: TestContainerCommit (0.40 seconds)
=== RUN TestOnContainerStart
--- PASS: TestOnContainerStart (0.27 seconds)
=== RUN TestOnContainerCreated
--- PASS: TestOnContainerCreated (0.04 seconds)
=== RUN TestOnContainerStop
--- PASS: TestOnContainerStop (0.42 seconds)
	container_test.go:175: Receieved exit code: 1
=== RUN TestCancelOnEvent
--- PASS: TestCancelOnEvent (2.39 seconds)
=== RUN TestRestartContainer
--- PASS: TestRestartContainer (0.66 seconds)
=== RUN TestListContainers
--- PASS: TestListContainers (1.48 seconds)
=== RUN TestWaitForContainer
--- PASS: TestWaitForContainer (10.26 seconds)
=== RUN TestInspectContainer
--- PASS: TestInspectContainer (0.18 seconds)
=== RUN TestRepeatedStart
--- SKIP: TestRepeatedStart (0.00 seconds)
	container_test.go:415: skip this until the build box issues get sorted out
=== RUN TestNewContainerOnCreatedAndStartedActions
--- PASS: TestNewContainerOnCreatedAndStartedActions (0.45 seconds)
=== RUN TestNewContainerOnCreatedAction
--- PASS: TestNewContainerOnCreatedAction (0.03 seconds)
=== RUN TestNewContainerOnStartedAction
--- PASS: TestNewContainerOnStartedAction (0.44 seconds)
=== RUN TestFindContainer
--- PASS: TestFindContainer (0.08 seconds)
=== RUN TestContainerExport
--- PASS: TestContainerExport (1.12 seconds)
=== RUN TestImageAPI
I1217 12:14:30.794379 00996 api.go:729] pushing image from repo: localhost:5000/testimageapi/busyb to registry: localhost:5000 with tag: 
I1217 12:14:33.460941 00996 api.go:743] finished pushing image from repo: localhost:5000/testimageapi/busyb to registry: localhost:5000 with tag: 
I1217 12:14:33.890418 00996 api.go:729] pushing image from repo: localhost:5000/testimageapi/busybox to registry: localhost:5000 with tag: victim
I1217 12:14:35.942231 00996 api.go:743] finished pushing image from repo: localhost:5000/testimageapi/busybox to registry: localhost:5000 with tag: victim
I1217 12:14:36.413307 00996 api.go:729] pushing image from repo: localhost:5000/testimageapi/busybox to registry: localhost:5000 with tag: latest
I1217 12:14:38.463287 00996 api.go:743] finished pushing image from repo: localhost:5000/testimageapi/busybox to registry: localhost:5000 with tag: latest
I1217 12:14:38.465604 00996 api.go:729] pushing image from repo: localhost:5000/testimageapi/busybox to registry: localhost:5000 with tag: snapshot
I1217 12:14:40.513293 00996 api.go:743] finished pushing image from repo: localhost:5000/testimageapi/busybox to registry: localhost:5000 with tag: snapshot
I1217 12:14:44.073311 00996 api.go:729] pushing image from repo: localhost:5000/testimageapi/busybox to registry: localhost:5000 with tag: latest
I1217 12:14:46.119918 00996 api.go:743] finished pushing image from repo: localhost:5000/testimageapi/busybox to registry: localhost:5000 with tag: latest
OK: 8 passed
--- PASS: TestImageAPI (21.80 seconds)
PASS
ok  	github.com/control-center/serviced/commons/docker	40.030s
```

![screen shot 2014-12-17 at 12 04 35](https://cloud.githubusercontent.com/assets/2837923/5476313/8031a83a-85e6-11e4-8c9f-cc13d2e0d89b.png)