Add ci scripts

[tmatsuo]

Adding some basic kokoro builds for now. We'll add more tests with #2 and #3.

[coryan]

This is fantastic, thank you for starting this work. I left a bunch of comments inline.
In addition:

• Can we move the .cfg files to ci/kokoro/docker/?
• The CI compiles the code, but I do not think we test the install target for our libraries. We should, but that can be in a future PR.

Note that testing install is more than just running make install, we also want to verify the installed headers and libraries are usable, i.e., we need to compile, link and run a test program against said libraries.

@scotthart just FYI, note how we now have 3 copies of the scripts to drive the CI builds. A better way does not immediately come to mind, but this is what I mean by we need to design that part of the multi-repository thing.

[tmatsuo]

@coryan Thanks, I think I addressed all the comments. PTAL

[coryan]

A couple of nits, please fix before merging.

Do we need a common.cfg file like this one?

https://github.com/googleapis/google-cloud-cpp/blob/master/ci/kokoro/docker/common.cfg

[tmatsuo]

@coryan Thanks, I added common.cfg, merging