Implement handling of Alertmanager config

[TheoBrigitte]

Towards: giantswarm/roadmap#3747

This PR implements the Alertmanager resources/job used to load the configuration from the secret (see #188) into Mimir's Alertmanager using Mimir's Alertmanager API.

Here is how it works:

• Read informations from the secret, using alertmanager.yaml key for the main configuration and any .tmpl key for templates
• Set each template name in the alertmanager config
• Compile the request
• Send a POST /api/v1/alerts request
• Handle any non 201 status code as error
• Et voila

Note:

• +414 changes in go.sum, that's because of github.com/prometheus/alertmanager/config import needed to load and mangle the alertmanager configuration