Add option to configure the Keycloak client timeout

[LoicGombeaud]

Our Keycloak instance is sometimes a bit slow to respond:

module.release_environment.keycloak_realm.realm: Creating...

Error: Post https://keycloak.is4.local/auth/admin/realms: net/http: request canceled (Client.Timeout exceeded while awaiting headers)

  on .terraform/modules/release_environment/main.tf line 5, in resource "keycloak_realm" "realm":
   5: resource "keycloak_realm" "realm" {

The request is cancelled, but the realm does get created, which leads to a conflict on subsequent terraform apply calls; and since the realm is not in the Terraform state, a terraform destroy does not solve the issue.

This PR allows for the Keycloak client timeout to be configurable, while keeping the default value of 5 seconds.

[LoicGombeaud]

The tests are failing, but the failures don't seem related to my changes

edit: all green now :)

[LoicGombeaud]

ping @mrparkers , what do you think about this?

[fharding1]

This LGTM, and I've had this (timeouts) happen to me when I'm on a slow network connection.

[mrparkers]

Sorry for the delayed review. This looks great, I was actually starting to run into issues with this myself.

Thanks for the contribution!