Add configuration option for libvirt endpoint

This was needed because we really have to be using 'qemu:///system' rather
than the default on CentOS, which is 'qemu:///session'
Update/clarify comments in haas.cfg.example.

Also, change the database uri to something more representative of a
production setup.
Merge pull request #342 from zenhack/update-haas.cfg.example

Update/clarify comments in haas.cfg.example.
Fix bug 316

If setup.py is called from anywhere other than the directory where
it resides, it is unable to locate requirements.txt (which exists
in the same directory as setup.py).

This patch makes it so setup.py can tell where it is being called
from and use this information to locate requirements.txt.
Use path.join
Merge pull request #341 from gsilvis/libvirt_endpoint

Add configuration option for libvirt endpoint
Merge pull request #344 from ryabouza/fix-bug-316

Improve setup.py's ability to locate requirements.txt
Update overview.md
Merge pull request #346 from Tweak-Man/patch-1

Correct a spelling mistake
Generate and ubuntu base image we can log into.

See the README. @lokI8's puppet manifests can be used on top of this,
though that needs testing.
Merge pull request #349 from zenhack/ubuntu-headnode

Generate and ubuntu base image we can log into.