Remove str() from LOG.* and exceptions

gettextutils is expecting to receive unicode strings
rather than basestrings. A basestring can cause an
unhandled exception in the logging code. To help avoid
such issues we should remove str() from LOG.* messages and
exceptions. We have verified that the %s formatting code
properly handle getting strings to unicode where necessary.

Copied from https://review.openstack.org/#/c/77722

Change-Id: I082af5c9ae8bf9859382c2c387b10b48358e10b3
Related-Bug: #1286306