Default net assignment

Adds default network assignment to Quark, along with yanking out
repoze.tm2 for compatibility while reinstating the standard
sqlalchemy-style transaction sessions to eliminate the race condition in
mass-assigning IP addresses under load.

Also fixes a typo with ipam_reuse_after which was previously, and
incorrectly, a boolean config value.
Merge pull request #168 from Cerberus98/transaction_merge

Default net assignment