Use iniset to set sqlalchemy.url

* In the alembic.ini file, the "iniset" function takes care of handling
  changes to ini files, so is better than using sed which can get out
  of date depending on the value stored.

Change-Id: If5c13555cd3956add5e91eb9f5b2e6f12a2fe557
Signed-off-by: Joe Mills <joe@midokura.com>