Add scheme to alertmanagers config section

[sed-i]

Issue

The alertmanagers section in prometheus's config is rendered without a scheme and without tls_config, and prometheus uses http by default.

Solution

• [x] Add scheme to alertmanager config.
• [ ] Figure out how to populate the tls_config section with a ca_file that matches alertmanager.

This PR goes in tandem with https://github.com/canonical/alertmanager-k8s-operator/pull/171.

Reference: https://prometheus.io/docs/prometheus/latest/configuration/configuration/#tls_config

Context

https://github.com/canonical/cos-lite-bundle/issues/75

Testing Instructions

• Pack alertmanager and prom from the two PRs
• Deploy and relate prom to alertmanager, and alertmanager to self-signed-certificates
• Confirm alerts are firing on prom's /api/v1/alerts and that the same alerts show in alertmanagers /api/v2/alerts.

Release Notes

Add scheme to alertmanagers config section.

[sed-i]

Itests passed but not reflected on the PR page here. Merging.