Add the sprig functions in the template engine

[terlenbach]

Description

This PR is the implementation of #1848. Basically it adds the sprig library (https://github.com/Masterminds/sprig) to the template engine.

[ldez]

Please read the contributing guide to know how to add dependencies.
https://github.com/containous/traefik/blob/master/.github/CONTRIBUTING.md#setting-up-glide-and-glide-vc-for-dependency-management

[ldez]

Please remove this, we don't manage dependencies like that: https://github.com/containous/traefik/blob/master/.github/CONTRIBUTING.md#setting-up-glide-and-glide-vc-for-dependency-management

[terlenbach]

👌

[ldez]

Could you do that:

1. Revert glide.yml, glide.lock and vendor changes. After that you must have nothing in the git stage.
2. Use the github.com/multiplay/glide-pin plugin: glide pin --exact
3. Commit the glide.yml with pinned version
4. Add dependency: ./script/glide.sh get YOUR/DEPENDENCY
5. Commit glide.yml, glide.lock and vendor
6. Revert the pin commit: git revert HASH_OF_PIN_COMMIT
7. Edit manually glide.yml to add the dependency
8. Use glide-hash to get the manifest file hash and put it in the lock file to please validate-glide.
9. Commit glide.yml and glide.lock
10. Squash all previous commit: git rebase -i HASH

[ldez]

please follow exactly my previous comment:

• first, revert all your changes on glide.lock, glide.yml and vendor (step 1)

[emilevauge]

Hi @thomasbach76, thanks for your great contribution!
Design LGTM

[ldez]

Thanks 👍

Could you add some explanations and a reference to sprig in our documentation?

[ldez]

n := len(configuration.Backends); n != 1 --> len(configuration.Backends) != 1

[ldez]

Great job!

LGTM!

[terlenbach]

Not sure why the semaphoreci check failed here...

[ldez]

It's not you, I have relaunch the build.

[juliens]

LGTM