Switch back to user-provided IAM key for Floaty

Exoscale's new IAM API (v3) isn't yet supported by the Terraform module, so we switch back to a user-provided IAM key for Floaty

This reverts #69 (commit c6938673f029a3cacebf57a555e05f10f385a38d), reversing changes made to 10e07d48fe8de339e2a6084b8e419763c1aeda49.

This PR also updates the Exoscale Terraform provider to v0.50.0, since we've updated the provider in the vshn-lbaas-exoscale module as well.

Replaces #81 #76

Checklist

[x] Keep pull requests small so they can be easily reviewed.
[x] Categorize the PR by setting a good title and adding one of the labels: bug, enhancement, documentation, change, breaking, dependency as they show up in the changelog
[x] Link this PR to related issues.
[x] Switch to tagged terraform-modules version