system-test: shard domain

* Test ShardDomain
* Improve error message when options cannot be set at location scope
** Used to say `"pagespeed" directive Option cannot be set at location
   scope`, now says `"pagespeed" directive "ShardDomain" cannot be set
   at location scope`.
* Marked `CustomFetchHeader`, `MapOriginDomain`, `MapProxyDomain,
  `MapRewriteDomain`, and `ShardDomain` as legal to set in location
  blocks.  In `mod_instaweb.cc` they're listed under `All two parameter
  options that are allowed in <Directory> blocks`.
Merge pull request #223 from pagespeed/jefftk-test-domain-sharding

system-test: shard domain