Invoke: Make settings file name handling more DRY
Invoke: Make settings file name handling more DRY

Instead of repeating hard-coded 'pelicanconf.py' values throughout
Invoke's task.py template, assign default settings file names to
variables, and use those variables where applicable.

iKevinY/pelican,talha131/pelican,iKevinY/pelican,farseerfc/pelican,ingwinlu/pelican,getpelican/pelican,farseerfc/pelican,ingwinlu/pelican,getpelican/pelican,avaris/pelican,talha131/pelican,avaris/pelican