"standby promote": make timeout values configurable

This introduces following new configuration file parameters, which
were previously hard-coded values:

 - promote_check_timeout
 - promote_check_interval

Implements GitHub #387.

doc/repmgr-standby-unregister.sgml

@@ -43,6 +43,22 @@
     </para>
   </refsect1>
 
+  <refsect1>
+    <title>Options</title>
+    <variablelist>
+
+      <varlistentry>
+        <term><option>--node-id</option></term>
+        <listitem>
+          <para>
+            <varname>node_id</varname> of the node to unregister (optional)
+          </para>
+        </listitem>
+      </varlistentry>
+
+    </variablelist>
+  </refsect1>
+
   <refsect1>
     <title>Event notifications</title>
     <para>