Fix isEnabledFor for compatibility with logging

After refactoring Keystone to use the unified logging implementation
from Oslo-incubator there were some issues using isEnabledFor with the
ContextAdapter provided in keystone/openstack/common/log.py. This
removes those calls and adds a check in keystone/common/wsgi.py to
handle the case where Keystone is running Python 2.6, in which
ContextAdapter doesn't have 'isEnabledFor'.

fixes bug 1213284

Change-Id: I8c64b12dcd9d955c96dc5a48348555ff37b15116
Merge "Fix isEnabledFor for compatibility with logging"