Add hacking check to enforce no client aliases

Adds hacking rule to prevent clients being defined using
"self.client" as a service alias. Doing so makes code difficult to
read and harder to maintain.

Change-Id: I060042d6af743079bdb43623e49dbfeba6f46fad
diff --git a/HACKING.rst b/HACKING.rst
index b1d730d..178e538 100644
--- a/HACKING.rst
+++ b/HACKING.rst
@@ -37,6 +37,8 @@
 - [P101] RBAC test filenames must end with "_rbac.py"; for example,
          test_servers_rbac.py, not test_servers.py
 - [P102] RBAC test class names must end in 'RbacTest'
+- [P103] ``self.client`` must not be used as a client alias; this allows for
+         code that is more maintainable and easier to read
 
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