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.

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

Change-Id: I7ce919cddfd6d6175ae67bd864f82e256ebc7090