Drop service* tables only if they exist

Tables servicedefinitions and servicetypes are not used by neutron but
created using db migration tool. If quantum-server in grizzly was
started without using migration tool then tables mentioned above are not
created. During migration from grizzly to havana alembic attempts to
drop these unused tables. This patch uses "IF EXISTS" clause when
dropping mentioned tables.

Closes-bug: #1273355
Change-Id: I33fcb3e1dc96ce37ec9c00987cb5a3a953ca691d
Merge "Drop service* tables only if they exist"