fixes CC-605: validate host before adding host

https://jira.zenoss.com/browse/CC-605

DEMO0 - unit test for validation:
```
~/src/europa/src/golang/src/github.com/control-center/serviced/validation
# plu@plu-9: go test -v
=== RUN Test
OK: 2 passed
--- PASS: Test (0.00 seconds)
PASS
ok  	github.com/control-center/serviced/validation	0.004s
```

DEMO1 - serviced host add with valid hostid succeeds:
```
root@ip-10-111-3-224:~# hostid
6f0ae003
root@ip-10-111-3-224:/opt/serviced/bin# serviced host add 10.111.3.224:4979 default
6f0ae003
```

DEMO2 - serviced host add with invalid hostid fails:
```
root@ip-10-111-3-224:~# hostid
00000000

root@ip-10-111-3-224:/opt/serviced/bin# serviced host add 10.111.3.224:4979 default
ValidationError: 
   0 -  not valid hostid: 00000000
```
DEMO3 - adding invalid hostid fails from UI:
![screen shot 2014-12-17 at 13 11 24](https://cloud.githubusercontent.com/assets/2837923/5477166/823c4c90-85ee-11e4-97c5-899a520a635c.png)