Don't break on bogus tags
Changed tag manipulation options

Removed the 'add_hostname_if_missing' option, and replaced it with two
more general-purpose options:

tags_if_missing - array of tags to add if they're not present
tags_override - array of tags to add/override unconditionally

The "host=os.Hostname()" can be added with either of these (tho the
hostname must be defined statically in the config).