Fix Keystone doc build errors with SQLAlchemy 0.9

The Keystone docs failed to build when using SQLAlchemy 0.9. Among
the errors that are reported is this one:

 sphinx.errors.SphinxWarning:
 /opt/stack/keystone/keystone/openstack/common/db/sqlalchemy/utils.py:
 docstring of keystone.openstack.common.db.sqlalchemy.utils.select:12:
 WARNING: undefined label: coretutorial_selecting (if the link has no
 caption the label must precede a section header)

To fix this, the `select` import was removed from
openstack.common.db.sqlalchemy.utils, and the callers were
changed to call sqlalchemy.sql.select.

Change-Id: I4995afc20c014f3a05aeff1d31a65b21fe279265
Closes-Bug: #1296333
Merge "Fix Keystone doc build errors with SQLAlchemy 0.9"