Update overview.md

Added headnode host as a component of a HaaS installation
Check that HNIC is attached before attaching to bridge

Previously, the code would attempt to attach the HNIC to a network, and hit a
null dereference if it wasn't attached.  Now it simply skips creating the
HNIC.  It might be nice to create the HNIC but not attach it, but it's a
little harder to do that.

This fixes bug 195
Merge pull request #196 from gsilvis/195

Check that HNIC is attached before attaching to bridge
Fixed a typo in a check_status_code invokation.
Merge pull request #197 from ryabouza/typoFix

Fixed a typo in a check_status_code invocation.
Use a fresh UUID for vm name, rather than SQL id

This makes the VM names globally unique, allowing multiple different instances
of the haas to share a headnode host.  It also fixes #123.
Add unique constraint to unique ID field
Merge pull request #198 from gsilvis/libvirt_id

Use a fresh UUID for vm name, rather than SQL id
Iterate over nodes rather than networks in project_apply

When iterating over networks, unattached ports won't be applied.  This created
a bug where nodes were never actually removed from networks.  Iterating over
the nodes instead ensures that every relevant port is configured.

This fixes #200
dell: Document surprising success of detaching port

While fixing #200, ryabouza and I tried fixing the problem here, figuring that
sending 'None' instead of a VLAN ID was clearly incorrect.  But it's actually
fine!
Merge branch 'remove-port' of https://github.com/gsilvis/haas into deployment-tests-vlan-fix
Added isolated network test
Finished network isolation test for deployment tests.
Fixed a bug in the hnic_cleaup decorator that prevented it from deleting the appropriate VLANs.