Make network_vlan an optional OpParam

Make network_vlan OpParam of OpNetworkConnect default to the empty
string and thus make it optional (see defaultField definition). This
way we keep backwards compatibility with old RAPI.

Additionally bypass a sphinx warning related to the generated html
(i.e. Inline literal start-string without end-string) in case an
empty string is used as default value.
Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>
Reviewed-by: Hrvoje Ribicic <riba@google.com>

lib/build/sphinx_ext.py

@@ -173,7 +173,10 @@ def _BuildOpcodeParams(op_id, include, exclude, alias):
     if has_default or has_test:
       buf.write(" (")
       if has_default:
-        buf.write("defaults to ``%s``" % (default,))
+        if default == "":
+          buf.write("defaults to the empty string")
+        else:
+          buf.write("defaults to ``%s``" % (default,))
         if has_test:
           buf.write(", ")
       if has_test:

src/Ganeti/OpParams.hs

@@ -1634,5 +1634,5 @@ pNetworkLink =
 
 pNetworkVlan :: Field
 pNetworkVlan =
-  withDoc "Network vlan when connecting to a group" $
-  simpleField "network_vlan" [t| String |]
+  withDoc "Network vlan when connecting to a group" .
+  defaultField [| "" |] $ stringField "network_vlan"