Add new feature: expand_env_vars

[oulman]

This PR adds support for expand_env_vars which will evaluate/expand environment variables passed to extra_env_vars in generic ad/kv2 secrets.

Example:

kv_secret "consul" {
    path = "services/terraform/consul_backend"
    target_provider = "generic"
    expand_env_vars = true
    attribute_map = {
        "CONSUL_HTTP_TOKEN" = "secretid"
    }
    extra_env_vars = {
        "CONSUL_HTTP_ADDR" = "consul.service.dc1.consul:8501"
        "CONSUL_CACERT" = "$HOME/consul-ca-cert.pem"
        "CONSUL_CLIENT_CERT" = "$HOME/consul-cli-cert.pem"
        "CONSUL_CLIENT_KEY" = "$HOME/consul-cli-key.pem"
        "CONSUL_HTTP_SSL" = "true"
    }
}

Produces:

CONSUL_HTTP_TOKEN=<SNIP>
CONSUL_CACERT=/Users/oulman/consul-ca-cert.pem
CONSUL_CLIENT_CERT=/Users/oulman/consul-cli-cert.pem
CONSUL_CLIENT_KEY=/Users/oulman/consul-cli-key.pem
CONSUL_HTTP_ADDR=consul.service.dc1.consul.server.ufl.edu:8501
CONSUL_HTTP_SSL=true
```