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Split out the hostname when default dhcp_domain is used in nova.conf
**What this PR does / why we need it**:
When /etc/nova/nova.conf does not have specify dhcp_domain to empty
string, a default string of '.novalocal' is returned by the meta data
service. So we need to just split the string and pick the first one
in the array.
```
$ curl http://169.254.169.254/latest/meta-data/hostname
testvm-1.novalocal
$ curl http://169.254.169.254/latest/meta-data/public-hostname
testvm-1.novalocal
$ curl http://169.254.169.254/latest/meta-data/local-hostname
testvm-1.novalocal
```
**Which issue(s) this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close the issue(s) when PR gets merged)*:
Fixes #
**Special notes for your reviewer**:
**Release note**:
```release-note
NONE
```