Add enable_snat column to routers table

* This change will add the enable_snat column to the routers table if
  it is missing.
* This will happen during operation, not during migration.

Fixes MN-1121
Signed-off-by: Joe Mills <joe@midokura.com>
Merge pull request #82 from JoeMido/add_column_v2

Add enable_snat column to routers table