configuration should allow generation of FTA names

[Jeff Inman]

... something like what we do with hosts, for example:

# supposing we have batch-FTAs named "fta02.my.org" and "fta03.my.org"
<bfta_pattern> "fta%02d.my.org" </bfta_pattern>
<bfta_offset> 2 </bfta_offset>
<bfta_count> 2  </bfta_count>

And then similarly for interactive-FTAs.

Then update marfs_config to parse this out, so it can be used by scripts. pipetool does already randomize its load across known FTAs, but this would allow it to automatically accommodate changes in the site architecture (or to run at other sites).