VNC harcoded server name removed

  VNC server can be other than WebSockify, so there is a need to
  configure it once in a while for various deployments

    - added config option
    - updated test

Change-Id: I938bae347eaa9fcf76e4118188b325ba6d3efa77
Closes-Bug: #1748339
diff --git a/tempest/config.py b/tempest/config.py
index 8a6370a..f374c75 100644
--- a/tempest/config.py
+++ b/tempest/config.py
@@ -404,6 +404,10 @@
                 default=False,
                 help='Enable VNC console. This configuration value should '
                      'be same as [nova.vnc]->vnc_enabled in nova.conf'),
+    cfg.StrOpt('vnc_server_header',
+               default='WebSockify',
+               help='Expected VNC server name (WebSockify, nginx, etc) '
+                    'in response header.'),
     cfg.BoolOpt('spice_console',
                 default=False,
                 help='Enable Spice console. This configuration value should '