keystone v3 is the default API version in horizon now,
so there is no need to specify it in local_settings.py explicitly.
This commit also makes minor changes in lib/horizon
_horizon_config_set().
* Do not insert a blank line after each setting.
* Use the local variable $file to specify the target file
consistently.
Change-Id: I5faea3e1f357726a256d2b48fc1afeabfead4998