Bugfix: Dovetail tool will crash when hosts.yaml file with incorrect format

Bugfix: Dovetail tool will crash when hosts.yaml file with incorrect format

If the hosts.yaml file is like,
—
hosts_info:
192.168.141.101:

it will raise an exception,
File "/usr/local/lib/python2.7/dist-packages/dovetail/utils/dovetail_utils.py",
line 227, in add_hosts_info
names=hostnames)
File "/usr/local/lib/python2.7/dist-packages/python_hosts/hosts.py",
line 54, in _init_
raise Exception('Address and Name(s) must be specified.')
Exception: Address and Name(s) must be specified.

If the hosts.yaml file is empty, it will crash with,
File "/usr/local/lib/python2.7/dist-packages/dovetail/container.py",
line 174, in create
if hosts_yaml['hosts_info']:
TypeError: 'NoneType' object has no attribute '_getitem_'

JIRA: DOVETAIL-621

Change-Id: I547928e514b3294c048379459c07df89879fbb03
Signed-off-by: xudan <xudan16@huawei.com>