Use tempfile.tempdir for lock_path if OSLO_LOCK_PATH is not set

DevStack spends a lot of time setting lock_path in a bunch of
locations.

http://codesearch.openstack.org/?q=oslo_concurrency%20lock_path&i=nope&files=&repos=

The default for lock_path is an environment variable
OSLO_LOCK_PATH.

http://codesearch.openstack.org/?q=OSLO_LOCK_PATH&i=nope&files=&repos=

Since the default is to consult an environment variable, which may not
even be set, this means that really there is no default. If you do
not set it - you end up getting errors like:

http://paste.openstack.org/show/488108/

Ideally - a library should have a reasonable default, where if a user
doesn't set something, the default will at least work. So, let's use
Python's built in tempfile module, which has fairly complex rules for
determining a sane directory to place temporary items in.

https://docs.python.org/2/library/tempfile.html#tempfile.tempdir

Change-Id: I6906af43bc0255cd215f2d9584ea000c81f5880e

oslo_concurrency/lockutils.py

@@ -41,11 +41,14 @@
                 help='Enables or disables inter-process locks.',
                 deprecated_group='DEFAULT'),
     cfg.StrOpt('lock_path',
-               default=os.environ.get("OSLO_LOCK_PATH"),
+               default=os.environ.get("OSLO_LOCK_PATH", tempfile.gettempdir()),
                help='Directory to use for lock files.  For security, the '
                     'specified directory should only be writable by the user '
                     'running the processes that need locking. '
                     'Defaults to environment variable OSLO_LOCK_PATH. '
+                    'If OSLO_LOCK_PATH is not set in the environment, use the '
+                    'Python tempfile.gettempdir function to find a suitable '
+                    'location. '
                     'If external locks are used, a lock path must be set.',
                deprecated_group='DEFAULT')
 ]