bugfix: hosts info is wrongly processed

bugfix: hosts info is wrongly processed

changes:
1. add usage example in hosts.yaml.sample
2. fix /etc/hosts from wrong format {'hostname': 'ip'} to
   the right 'ip hostnames'
3. fix --add-hosts from wrong usage --add-host {'hostname': 'ip'} to
   the right --add-host='hostnames':ip
4. support per ip mapping mutiple hostnames, which is also
   in line with /etc/hosts schema

JIRA: DOVETAIL-556

Change-Id: I92e8cb0fd3476b61f5c73a69e34c4a66aef51d6e
Signed-off-by: SerenaFeng <feng.xiaowei@zte.com.cn>