Allow user to specify/override per-command host list via command arguments.
Allow user to specify/override per-command host list via command arguments.

This way, one can easily fire off a host-agnostic command and specify the hosts
"at runtime".

Also removed unnecessary subroutine _execute_command() as it's used in one place and is only two lines long =)

bitprophet/fabric,kmonsoor/fabric,xLegoz/fabric,raimon49/fabric,kxxoling/fabric,likesxuqiang/fabric,pashinin/fabric,felix-d/fabric,tolbkni/fabric,opavader/fabric,haridsv/fabric,bitmonk/fabric,fernandezcuesta/fabric,StackStorm/fabric,mathiasertl/fabric,getsentry/fabric,tekapo/fabric,askulkarni2/fabric,rane-hs/fabric-py3,TarasRudnyk/fabric,itoed/fabric,pgroudas/fabric,ploxiln/fabric,SamuelMarks/fabric,cmattoon/fabric,sdelements/fabric,cgvarela/fabric,jaraco/fabric,hrubi/fabric,qinrong/fabric,ericholscher/fabric,akaariai/fabric,elijah513/fabric,bspink/fabric,simon-engledew/fabric,MjAbuz/fabric,amaniak/fabric,rbramwell/fabric,rodrigc/fabric