BigSwitch: Don't import portbindings_db until use

BigSwitch: Don't import portbindings_db until use

The Big Switch ML2 driver re-uses large parts of
code from the Big Switch plugin. However, the plugin
uses the old portbindings_db in its port tracking
code. This db is not created/used by ML2, so it does
not exist when the Big Switch ML2 driver is being used.
This caused a problem when the portbindings_db module
would be imported even though it wasn't being used
since the SQLAlchemy models would create the
relationships during the import.

This patch moves the portbindings_db import down to
the methods that leverage it so it won't be imported
during use by the ML2 driver.

This is a temporary solution which can be back-ported
if disabling auto schema generation is back-ported
to icehouse. An appropriate fix will require some
refactoring to avoid the port tracking code from
being imported for ML2.

Closes-Bug: #1300628
Change-Id: I551abf9d6ec832c423cba0d50bb27715b43c0b1c