[New function] Add get and mod hostname in network module

I add a new function to network.py,like this:
```
[root@saltmaster ~]# salt 'node1' network.get_hostname 
node1:
    saltmaster.itnihao.com
[root@saltmaster ~]# salt 'node1' network.mod_hostname  salt.control.com
node1:
    True
[root@saltmaster ~]# salt 'node1' network.get_hostname                  
node1:
    salt.control.com
```

https://github.com/itnihao/salt/commit/43d74142e359a64d23dfb0c4dbae22d78ab02299

This is work on RHEL(CentOS)  ok.