Do not clean docs directory

The docs/source has been introduced by change-id:
Ide5f7d980a294c7a9f8a3decaed0939f1c239934, but clean.sh try to clean
whole docs direcotry, which is not right.

Meanwhile, there are directories in FILES_TO_CLEAN variable, which means
rm -f will not work. We need to use rm -rf.

Change-Id: I4e10d70d5638ce7e7a9215f5bd848422aa90aad3
Closes-Bug: #1335399
Merge "Do not clean docs directory"
Merge branch 'master' of github.com:openstack-dev/devstack