move tmp dir from edx-west/tmp to edx-west/ec2_cache

Per feedback from @feanil on PR #849, want our release branch to line up
with what we're pulling into master.

Going back to putting ssh connections in /tmp.  They have globally
unique names, putting in /tmp is consisten with other things, and having
them live in a dir named ec2_cache feels odd now.
Merge pull request #1057 from edx/sef/edx-west/local_playbook_tempdir

Stanford only: move tmp dir from edx-west/tmp to edx-west/ec2_cache