Save VM credentials in config to be able to reconnect to slaves

Currently the plugin takes VM credentials via JClouds, using NodeMetadata.getCredentials. This works when a VM is just created, but if there's a need to reconnect to VM and do bootstrap again (like when restarting Jenkins), this doesn't work as JClouds doesn't provide credentials at that time, as confirmed by NodeMetadata.getCredentials javadoc:

```java
   /**
    * If possible, these are returned upon all detail requests. However, it is often the case that
    * credentials are only available when a node is initially created.
    * 
    * @see ComputeServiceContext#credentialStore
    */
   @Nullable
   LoginCredentials getCredentials();
```

To fix this I'm proposing to store credentials as a part of plugin configuration. The fix is tested with OpenStack, using openstack-nova provider.