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/
Migrate ensime_gen.py, reporting_server.py to new options format.

Testing Done:
manually invoking tasks w/ command line flags.
ci is running

Bugs closed: 746

Reviewed at https://rbcommons.com/s/twitter/r/1251/
fix signature in 'goal targets' and BUILD dictionary

We construct the arg signature for Targets by walking up the class inheritance chain, accumulating info. But if more than one class in the chain mentions a param, then... we were showing it more than once in the arg signature, whoops. This had other side effects: we doc'd some params we meant to hide.

With this change, the Target-class-walking code got big enough such that I separated it out into its own function.

Testing Done:
https://travis-ci.org/pantsbuild/pants/builds/39509603

Reviewed at https://rbcommons.com/s/twitter/r/1253/