Move to Native Testlets

[Mierdin]

This PR introduces native testlet capabilities - that is, the idea of providing functionality for the tests themselves, in native-go testlets, that are integrated at install time. A summary of changes are as follows

• Testlet infrastructure, mainly for encouraging compatibility with the testlet standard. At first, the native testlet approach was going to have all testing functionality built in to the todd agent, but since then, the decision was made to keep them as separate binaries. However, some of the infrastructure here (such as the Testlet interface) was maintained to encourage compatibility.
• Removal of the ping testlet, as that has been implemented here. The other bash testlets will be phased out in future PRs as their functionality is replaced by native Go testlets in the same way.
• The testlets themselves are in separate repositories, and are only brought into the system when the install for ToDD proper is run. This is done via gettestlets.sh
• Scripts to automate the download and installation of native testlet code. This is also helpful during development of testlets - though more work will be done in a future patch in this area.
• Various documentation updates. It's important to note that many of these changes will be overwritten by a revamp to the documentation that is currently in a branch, and will be addressed after this PR.

This patch existed for a long time, and saw several changes in direction for this approach. Forgive the at-times chaotic nature of the changes below. Some of the changed files are only loosely with the main purpose of this patch.

This also satisfies https://github.com/Mierdin/todd/issues/87

[Mierdin]

I believe I've addressed your comments.

[vcabbage]

General note, this could be changed to use exec.CommandContext (added in 1.7) and simplify the timeout logic below.

ctx, cancel := context.WithTimeout(context.Background(), time.Duration(ett.TimeLimit) * time.Second)
defer cancel()
cmd := exec.CommandContext(ctx, testletPath, thisTarget, tr.Args)

[Mierdin]

I created https://github.com/Mierdin/todd/issues/93 to track this so that when I move CI to Go 1.7, I'll add this. Thanks

[vcabbage]

os.IsNotExist(err) will only be evaluated if err is nil, in which case os.IsNotExist will always return false.

[Mierdin]

Yeah, my intention was that regardless of the error, I wanted to treat it like the testlet didn't exist. However, that's fairly confusing.

I clarified this a bit by making the error more generic (and removing os.IsNotExist(err) in Mierdin@6f1db24 - stating in the error that there was simply a problem accessing the testlet