To simplify the launchpadlib dependency installation for the
storyboard-migrate utility, use a separate tox testenv specifically
for this purpose and put that as its only additional runtime
requirement. Name the testenv "lpimport" with the expectation that
in time we may also rename the storyboard-migrate utility similarly
(so as to disambiguate it from the Alembic migrations we rely on
for database schema upgrades).
Change-Id: I8bd36bf69c235c8957ae4a161fd22cbe80028cf4
e386f32566