When overrides are not YAML valid, tdp_vars directory is inconsistent

[rpignolet]

If a YAML in a tdp_vars_overrides dir supply with --overrides option of tdp init is not valid, the tdp_vars dir is created and provision only with defaults but without overrides which is inconsistent. The user must remove tdp_vars dir manually before running again tdp init --overrides ....

We should validate YAML files before writing inside tdp_vars dir in order to not create tdp_vars dir and the content if a YAML file is not valid.