feat(constraint): Supports kv backends

[samber]

Related to #311

• BoltDB
• Consul KV
• Zookeeper
• Etcd

[keis]

In this one the check is triggered from the template and while not inheritly bad I think that for the sake of consistency the filter should happen before the template like in the others.

[emilevauge]

@samber thanks a lot :)
I tend to agree with @keis on the filtering :)

[samber]

Corrected ;-)

[vdemeester]

LGTM 🐋

[emilevauge]

@samber, thanks a lot :)

• There is an issue in toml documentation on constraints: traefik.toml constraints error: Expected map but found 'string'. #451 (comment)
• traefik.sample.toml should be updated too

Other than that, looks great :)

[emilevauge]

Hi @samber, I'm planning to get this PR in the 1.1 releases. Do you think it's possible ?

[emilevauge]

PR carried in #689 because of inactivity.