VirES WPS fixes

[pacesm]

This PR introduces a couple of fixes to the WPS client:

• POST requests are now sent with the proper application/xml content type header. The server may start enforcing request with the proper content type in the future.
• The response references may contain href attribute with the namespace prefix xlink:href. The used WPS 1.0 standard seem to be somewhat ambiguous which one is correct and therefore, to make the parser more robust, both variants are accepted.
• Retry requests for the failed status update have been implemented. The client now repeats the failed job status polling 3 times with 20 seconds interval.
• In order to be able to inform users about the failed requests, the default logging level has been changed from NO_LOGGING to ERROR.
• The client now handles response references as relative URLs (encountered in local development environment).