Revert "default run as root user because ``erlexec`` needs HOME env to be set"

This reverts commit ce91c65a7766fe784f9f87a9be59e47d7551e813.

Already set default user to run rabbitmq cmds in rabbitmq module. This
allow to use arbitrary user to run those cmds but still has a sane
default than 'root' (set as 'root' may not work when salt-minion not run
as 'root').
add runas parameter for all functions
pep8