Support single quoted Environment variables

[jevans-ccc]

Add support for Environment lines containing single quoted parts, e.g.

Environment="VAR1=word1 word2" VAR2=word3 "VAR3=$word 5 6" 'VAR4=7 8'
Environment='VAR5='
Environment='VAR6=9 10'

The specification permits double, single or unquoted parts, but previously only double quoted or unquoted parts were supported in systemctl.py.