DevServer: Add a new devserver call to list all control files' content.

Now when a suite job is created, it calls devserver to list all control
files' path, and then call devserver X times to get the content of each
control file, where X equals to the length of the control file list.
Since a suite job may fetch more than 1000+ control files, and all the
thouands of calls are made over ssh. It's significantly slower than
previous directly calling devserver by http.

The new call 'list_suite_controls' goes through the lists of all control files
for a suite and a build, and return the contents of these control files.

BUG=chromium:602562
TEST=local run devserver
curl
http://127.0.0.1:8082/list_suite_controls?suite_name=suite_attr_wrapper&build=veyron_speedy-paladin/R52-8182.0.0-rc3
expected return:
["# Copyright control_file_content1", ..., "# Copyright
control_file_contentX"]

Change-Id: Id5e46e31212eda98d0e044be20cf657985f985bf
Reviewed-on: https://chromium-review.googlesource.com/338931
Commit-Ready: Xixuan Wu <xixuan@chromium.org>
Tested-by: Xixuan Wu <xixuan@chromium.org>
Reviewed-by: Dan Shi <dshi@google.com>
1 file changed