Improve hostlist parsing
Improve hostlist parsing

Avoid looking up objects like DNS domains or the default DNS environment
multiple times.

Change-Id: I8bba9e8df49456728ad0030f550e29501b72d865
Reviewed-by: Priya Joshi <3c6cfa6f57440adfc837c4ad04483313acbc2582@morganstanley.com>

guillaume-philippon/aquilon,quattor/aquilon,quattor/aquilon,quattor/aquilon,guillaume-philippon/aquilon,guillaume-philippon/aquilon