EUCA-4877: Fix euca_conf config file manipulation

There were various problems with euca_conf's attempts to manipulate
config values, specifically in cases where a value occurred in
comments and also in an uncommented line. Each save operation now
rewrites the file in a way that better preserves comments and avoids
duplicate uncommented values. Config file merging is also improved
with this change.
Backport euca-validator into eucadmin