Deprecate SCREEN_LOGDIR in favor of LOGDIR

This is the first step in the log file cleanup.  If SCREEN_LOGDIR
is still set, symlinks will be created in the old screen log directory
so things like the devstack-gate log collector continues to work.

bp:logging-and-service-names
Change-Id: I3ac796e322a18dbd0b8b2310a08310ca159d7613
Merge branch 'nonovnc' into dt-test
Merge branch 'identity-v3-default' into dt-test
Merge branch 'conf-tempest' into dt-test
Merge branch 'test-service-functions' into dt-test
Merge branch 'exercise-refresh' into dt-test