allow relative path to parent to be configured

surprisingly (to me) this _is_ permitted in maven; according to the docs it only restricts:

> Properties are not supported in the following XPath elements:
> `/project(/parent)?/(groupId/artifactId/version)`
diff --git a/pom.xml b/pom.xml
index 4a223d6..525199c 100644
--- a/pom.xml
+++ b/pom.xml
@@ -25,7 +25,7 @@
         <groupId>org.apache.brooklyn</groupId>
         <artifactId>brooklyn-parent</artifactId>
         <version>1.0.0-SNAPSHOT</version>  <!-- BROOKLYN_VERSION -->
-        <relativePath>../brooklyn-server/parent/</relativePath>
+        <relativePath>${brooklyn.ui.relativePath.to.brooklyn.server.parent}</relativePath>
     </parent>
 
     <groupId>org.apache.brooklyn.ui</groupId>
@@ -84,6 +84,8 @@
         <brooklyn.version>1.0.0-SNAPSHOT</brooklyn.version><!-- BROOKLYN_VERSION -->
         <build.version>${revision}</build.version>
         <build.name>Apache Brooklyn</build.name>
+        <brooklyn.ui.relativePath.to.brooklyn.server.parent>../brooklyn-server/parent/</brooklyn.ui.relativePath.to.brooklyn.server.parent>
+
         <buildnumber-maven-plugin.version>1.4</buildnumber-maven-plugin.version>
 
         <!-- versions from brooklyn server which have a different var name here -->