Fix error when /etc/hostname contains a short hostname on Debian

[jleroy]

This pull request fix an error which occurs when /etc/hostname contains a short hostname (no domain name) on Debian.

$ cat /etc/hostname
salt-master

Error returned:

          ID: hostname
    Function: network.system
      Result: False
     Comment: An exception occurred in this state: Traceback (most recent call last):
                File "/usr/lib/python2.7/dist-packages/salt/state.py", line 1371, in call
                  **cdata['kwargs'])
                File "/usr/lib/python2.7/dist-packages/salt/states/network.py", line 380, in system
                  new = __salt__['ip.build_network_settings'](**kwargs)
                File "/usr/lib/python2.7/dist-packages/salt/modules/debian_ip.py", line 1487, in build_network_settings
                  domainname = sline[1]
              IndexError: list index out of range
     Changes:   

I didn't find where to add tests for network module.

[ghost]

Test PASSed.
Refer to this link for build results: http://jenkins.saltstack.com/job/salt-pr-build/3371/