Auto-detection for vCenter always return true

[Koji Nakazono]

[How to reproduce]
Execute the monasca-setup command on the machine on which vCenter is not configured.

[Expected Result]
Configuration file for vCenter check is not created.

[Observed Result]
Configuration file for vCenter check is created.
So the following logs are output each collector run.
collector.log
-----------
2016-05-26 03:34:13 EDT | WARNING | collector | monasca_agent.collector.checks.check.vcenter(vcenter.py:307) | vCenter not configured
2016-05-26 03:34:13 EDT | WARNING | collector | monasca_agent.collector.checks.check.vcenter(vcenter.py:307) | vCenter not configured
-----------

Currently, the detect method always returns True.
https://github.com/openstack/monasca-agent/blob/master/monasca_setup/detection/plugins/vcenter.py#L39
I think that this method should check whether the apropriate agrs exist or not when it cannot find the configuration file.