Renaming a user name with an existing user name

Change error message when we try rename user
name with an existing user name.

Change-Id: I2dbf9fee3c682a70c77544a8dd6b925311a3f019
Closes-Bug: #1389132
Imported Translations from Transifex

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https://wiki.openstack.org/wiki/Translations/Infrastructure

Change-Id: Idb2ece7aa586290445e233e6987841761e4962e2
Merge "Renaming a user name with an existing user name"
Merge branch 'master' of https://github.com/openstack/horizon