This commit introduces a new variable, horizon_keystone_url, which
allows the administrator to specify the Keystone URL.
This defaults to the internal Keystone URL for backwards compatibility's
sake.
Closes-Bug: #1759623
Change-Id: Idf178a6398000fcb2d02b6f37b8ef408218b94ee
Signed-off-by: Nick Jones <nick@stackhpc.com>