Add mysql support for ceilometer storage backend in devstack

Currently, devstack only support mongodb as ceilometer storage backend,
this patch is to add mysql storage support for ceilometer.
If you want to use mysql as backend during developing, you can specify
CEILOMETER_BACKEND=mysql in localrc file.
If you use mongodb, just ignore the parameter.

Change-Id: Ic2f475a9baa6d71a43cd29a6ca777ac972e47b0a
Implements: blueprint ceilometer-mysql-support
Use unique build dir for pip installs

There is a bug in pip [1] where it will choose to install a package
from an existing build-dir if it exists over the version actually
requested.

Thus if a prior component has installed a later version of the
package, the unpacked code is already in /tmp/$USER-pip-build; it gets
re-installed and manifests in a confusing error along the lines of

---
 Downloading/unpacking requests>=1.1,<1.2.3
   (from -r /home/stack//python-cinderclient/requirements.txt (line 5))
   Running setup.py egg_info for package requests
   Requested requests>=1.1,<1.2.3 (from -r
   /home/stack/python-cinderclient/requirements.txt (line 5)),
    but installing version 1.2.3
...
  error: Installed distribution requests 1.2.3 conflicts with
    requirement requests>=1.1,<1.2.3
---

I believe pip 1.4 fixes this problem, but it should always be safe to
specify a unique build-directory for pip installs to avoid picking up
old versions.

We also add a cleanup_tmp function for clearing out anything that
stack.sh might leave around when un-stacking, and add a catch-all for
the pip-build dir.

[1] https://github.com/pypa/pip/issues/709

Change-Id: I7ce919cddfd6d6175ae67bd864f82e256ebc7090
Merge "Add mysql support for ceilometer storage backend in devstack"