Fix the regression introduced in https://rbcommons.com/s/twitter/r/1186/

The original code in benchmark_run.py was:

  option_group.add_option(mkflag("target"), dest="target_class", action="append",
                            help="Name of the benchmark class.")
  ...
  ...
  self.caliper_args = self.context.options.target_class
  self.caliper_args += [ str1, str2 ]
  self.caliper_args.extend([str3, str4])
  ...
  ...
  # eventually deep in the callstack, a maybe_list(caliper_args) is called.

For whatever reason, we used "append" and the target_class is a list (even though
it should be a single string argument). But anyhow, it works, since the it was an
assignment "self.caliper_args = self.context.options.target_class"

Now the change introduced in the RB, while keeping the target still a list, it
changes the line "self.caliper_args = self.context.options.target_class" to

  self.args.insert(0, self.get_options().target)

Note even though it's called "target", it's actually an list. Thus we're inserting
a list as an element into a list. Later the maybe_list failed.

- also fixed the pants.ini benchmark setting name mismatch.

Testing Done:
local benchmark run worked.

CI (with all 3 commits) is baking: https://travis-ci.org/jinfeng/jinfeng-pants-fork/builds/39519055

Reviewed at https://rbcommons.com/s/twitter/r/1254/
Sync of https://github.com/pantsbuild/pants/commit/0ee4533aa745d7c48fff14dc839ed08bea499f8e