Compact pre-Icehouse database migrations <= 170.

Compacts the pre-Icehouse database migrations <= 170 into a
single migration (170_havana.py).

Pre-Icehouse users will now need to upgrade to Havana before
running any Icehouse migrations.

Tested on MySQL and PostgreSQL schemas on Fedora 19 to ensure the
new compacted migration generates an identical schema to the
previous migrate scripts in Havana.

Specific changes include:

1) removed 161_fix_system_metadata_none_strings.py. Nothing to
do here since this was a data conversion (no schema changes)

2) Removed some placeholder migrations for the Grizzly stable
branch.

Change-Id: I94a080ca122af009304975044f33c5e5192834ab
Implements: blueprint compact-havana-db-migrations
Merge "Compact pre-Icehouse database migrations <= 170."