[lager] Set SYSLOG_IDENTIFIER=application, fixed bug in PRIORITY=

Summary:
SYSLOG_IDENTIFIER=application
PRIROITY=level_to_num
PRIOTITY was constant before

Test Plan: Use lager_journald_backend and journalctl

Reviewers: skattelmann, orixmann, hwinkel, hsychla

Reviewed By: skattelmann

CC: druss, aschultz

Maniphest Tasks: T238

Differential Revision: https://review.tpip.net/D1035