Configurable temporary directory for tests

The temporary directory used for testing was always
keystone/tests/tmp. This change makes it so that the temporary
directory can be configured by setting the KEYSTONE_TEST_TEMP_DIR
environment variable. For example:

 $ mkdir ~/keystone_test_temp
 $ sudo mount -t tmpfs -o size=16M tmpfs ~/keystone_test_temp &&
    sudo chown $USER: ~/keystone_test_temp
 $ KEYSTONE_TEST_TEMP_DIR=$HOME/keystone_test_temp tox -e py27

Change-Id: I41f86a23882a382ab19747e008720c0da5fd56e6
Merge "Configurable temporary directory for tests"