bootstrap-install: Changed installer to remove old seed_mec sudoers(1) rule

Instead of doing nothing, remove the old rule and add the new one to
sudoers(1) file.

This modification corrects an error which the previous agent version caused
in outdated systems (e.g. LE 3, Ubuntu 8), making the sudoers(1) file
unreadable.

Signed-off-by: Erik Alexandre Pucci <eap08@c3sl.ufpr.br>
Acked-by: Adriano da Luz <al08@c3sl.ufpr.br>
Acked-by: Bruno Ribas <ribas@c3sl.ufpr.br>