Add cover/ to .gitignore

When we run "tox -e cover" to generate coverage report, testr will make
"cover" directory which contains the same html in "covhtml" made by
"./run_test.sh --coverage".  So, make git ignore this directory.

Change-Id: I829667a1ceb55418a5695545ad543a53f63821ba
Merge "Add cover/ to .gitignore"