fixes CC-605: validate host before adding host

https://jira.zenoss.com/browse/CC-605

DEMO - 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
```

DEMO - serviced started on valid hostid:
```
root@ip-10-111-3-224:~# hostid
6f0ae003

root@ip-10-111-3-224:~# tail -F /var/log/upstart/serviced.log
I1217 00:36:58.861746 12889 daemon.go:516] Trying to discover my pool...
W1217 00:36:58.864192 12889 daemon.go:526] masterClient.GetHost 6f0ae003 failed: hosts_server.go host not found (has this host been added?)

root@ip-10-111-3-224:/opt/serviced/bin# serviced host add 10.111.3.224:4979 default
6f0ae003
```

DEMO - serviced will not start with invalid hostid:
```
root@ip-10-111-3-224:~# hostid
00000000

root@ip-10-111-3-224:~# tail -F /var/log/upstart/serviced.log
...
I1217 00:29:02.967209 09395 host.go:144] building with ipsAddrs: [10.111.3.224] [1]
I1217 00:29:02.967700 09395 utils.go:133] looking for '10.111.3.224'
E1217 00:29:03.050558 09395 daemon.go:509] invalid hostid: 00000000
...
W1217 00:29:03.052897 09395 daemon.go:526] masterClient.GetHost 00000000 failed: hosts_server.go host not found (has this host been added?)

root@ip-10-111-3-224:/opt/serviced/bin# serviced host add 10.111.3.224:4979 default
invalid hostid for host: &{ID:00000000 Name:ip-10-111-3-224 PoolID:default IPAddr:10.111.3.224 RPCPort:4979 Cores:8 Memory:64426311680 PrivateNetwork:172.17.0.0/255.255.0.0 CreatedAt:0001-01-01 00:00:00 +0000 UTC UpdatedAt:0001-01-01 00:00:00 +0000 UTC IPs:[{HostID:00000000 IPAddress:10.111.3.224 InterfaceName:eth0 MACAddress:12:a9:d1:36:53:3e}] KernelVersion:#63-Ubuntu KernelRelease:3.13.0-36-generic ServiceD:{Version:1.0.0 Date:Wed Dec 17 00:27:07 UTC 2014 Gitcommit:2e995c9 Gitbranch:feature/CC-605 Giturl: Buildtag:0} MonitoringProfile:{MetricConfigs:[] GraphConfigs:[] ThresholdConfigs:[]} VersionedEntity:{DatabaseVersion:0}}

```