Add conf flag to switch new features for all agents and configurations.

This branch introduces new configuration section "features" to all configuration files and it manages the newly added features to enable/disable at runtime.

Please check if you are new to the concept.
http://webdevnights.github.io/feature-flipping/

dcmgr.conf will be changed as following:

```
features {
  openvnet true
}


amqp_server_uri "amqp://......"
```

The existing configuration classes have been changed to inherit from Dcmgr::Configurations::Features. The base class gives the read only parameter for the entire code base to switch the feature.

```
if Dcmgr::Configurations.dcmgr.features.openvnet
  # Do something for openvnet integration.
end
```