Allow overriding the workspace directory in prepare-workspace

The prepare-workspace role currently assumes that we are going to copy
our code into the home directory of the user. This may not always be the
case and so allow overriding this variable.

Change-Id: I6d640015dc764770f8a468edbcf7ec2ea3421e20

roles/prepare-workspace/defaults/main.yaml

@@ -0,0 +1 @@
+zuul_workspace_root: .

roles/prepare-workspace/tasks/main.yaml

@@ -4,6 +4,6 @@
 
 - name: Synchronize src repos to workspace directory.
   synchronize:
-    dest: .
+    dest: "{{ zuul_workspace_root }}"
     src: "{{ zuul.executor.src_root }}"
   no_log: true