Use Keystone's default token format if not set

Devstack was setting its own default for Keystone's token format, so
when Keystone's default token format changed then devstack needed to
be updated. With this change, devstack will only override Keystone's
token format if KEYSTONE_TOKEN_FORMAT is set explicitly. PKI setup
is assumed to be needed unless the KEYSTONE_TOKEN_FORMAT is set to
UUID.

Change-Id: Idfa78e93abd80273dadcf37007a024bb6a783a48
Merge "Use Keystone's default token format if not set"