Adds more uniformity to identity update_user calls

 * It was not possible to change a user's name using the KVS driver.
 * The KVS driver was not filtering the user dict before handing it
   back to that caller.
 * Adds a test to the LDAP driver to explicitly show that changing a
   user's name is not allowed.

Related-Bug: #1214686
Change-Id: Ia0951a58ef09833849d3eb5e714c7efbd901f5c5
Merge "Adds more uniformity to identity update_user calls"