Adding log message for keystone API mismatch

When the Horizon settings file specifies a v2.0 endpoint and
indicates v3 API to use with Keystone, the code redirects the
v2.0 to v3 without notice. Logging a warning, so deployers can
address the mismatch.

Closes-Bug: #1291457
Change-Id: If9e9e40af5ac23e8dea552d2d1a04597c67837a7