replace use of 'unicode' builtin
replace use of 'unicode' builtin

The builtin type 'unicode' does not exist under python 3. The runtime
check here is already looking at the python version, but the linter
does not perform the same check and it fails because the name is not
defined. Replace the explicit use of unicode here with six.text_type
so that the linter is happy.

Change-Id: I22a13fddaed1338c0f3e670d199bb6aa5f99bbf7
Signed-off-by: Doug Hellmann <0798ebecfb7bc4dfb9d0445c2fa030eb64e1fa1c@doughellmann.com>

openstack/monasca-log-api,openstack/monasca-log-api,stackforge/monasca-log-api,stackforge/monasca-log-api,stackforge/monasca-log-api,openstack/monasca-log-api