www/py-graphite-api: clean up rc.d script

• fix ordering in rc.d script so that daemon doesn't hang on startup
• teach rc.d to behave when installing with multiple python & gunicorn versions
• clean up substitutions in SUB_LIST, pkg-list and *.in script
• ensure working directories are re-created on startup if required

PR: 223264
Submitted by: Horst Kapfenberger <horst.kapfenberger@agoracon.at>
Approved by: jrm
MFH: 2018Q3
Differential Revision: https://reviews.freebsd.org/D13581