mongodb charm unit test has minor lint on import order

The path.append in the log_rotate test may not be necessary, as hooks is imported in other unit_tests without it. But if it is, we need to re-order it to satisfy E402.

Test automation just started surfacing import order lint errors on this file from 2014. Perhaps a flake8 rev, as this charm lints with the bleeding edge of flake8.

http://bazaar.launchpad.net/~charmers/charms/trusty/mongodb/trunk/view/head:/unit_tests/test_write_log_rotate_config.py#L7

00:00:51.533 ${OSCI_ROOT}/tools/make_exec.py Makefile "@flake8" "/flake8" |& tee $WORKSPACE/${test}-test.$BUILD_NUMBER; ret_code=${PIPESTATUS[0]}
00:00:52.046 INFO:root:Workspace dir: /var/lib/jenkins/workspace/charm_lint_check
00:00:52.046 INFO:root:Reading file: Makefile
00:00:52.047 INFO:root:Searching for: ['@flake8', '/flake8']
00:00:52.048 INFO:root:command: make -f Makefile lint
00:00:52.061 .venv/bin/flake8 --exclude hooks/charmhelpers actions actions/dump actions/perf actions/restore hooks tests unit_tests
00:00:52.673 unit_tests/test_write_log_rotate_config.py:7:1: E402 module level import not at top of file
00:00:52.700 make: *** [lint] Error 1
00:00:52.700 ERROR:root:Make target returned non-zero.
00:00:52.731 Returned: 1
00:00:52.731 ! Lint test failed.
00:00:52.736 ====> end lint test <=======================