Deprecate SCREEN_LOGDIR

SCREEN_LOGDIR is not really necessary, logging should just go to
LOGDIR.  For now, continue to honor it if it is set but only
symlinks to the actual log files will be present.

Change-Id: I4f7b641602acd896260b947dd270c07570e4c9d9
Merge branch 'cinder-backend-netapp' into dt-test
Merge branch 'cinder-backend-solidfire' into dt-test
Merge branch 'screen-log' into dt-test