Support TLS Version as an Option within SMTP (#2268)

In order for NervesHub to work against AWS SES, we need to be able to
set TLS Version to 1.2. The default that happens with Swoosh isn't
supporting what SES expects. This change permits the option to set
TLS_VERSION or leave it to default within the Swoosh

Author: jclarkgp

config/runtime.exs

@@ -370,6 +370,13 @@ if config_env() == :prod do
   config :swoosh, local: false
 
   if System.get_env("SMTP_SERVER") do
+    tls_versions =
+      System.get_env("SMTP_TLS_VERSIONS", "")
+      |> String.split(",")
+      |> Enum.map(&String.to_atom/1)
+
+    tls_opts = if Enum.any?(tls_versions), do: [versions: tls_versions], else: []
+
     config :nerves_hub, NervesHub.SwooshMailer,
       adapter: Swoosh.Adapters.SMTP,
       relay: System.fetch_env!("SMTP_SERVER"),
@@ -379,15 +386,16 @@ if config_env() == :prod do
       auth: :always,
       ssl: System.get_env("SMTP_SSL", "false") == "true",
       tls: :always,
-      tls_options: [
-        verify: :verify_peer,
-        cacerts: :public_key.cacerts_get(),
-        depth: 99,
-        server_name_indication: String.to_charlist(System.get_env("SMTP_SERVER")),
-        customize_hostname_check: [
-          match_fun: :public_key.pkix_verify_hostname_match_fun(:https)
-        ]
-      ],
+      tls_options:
+        [
+          verify: :verify_peer,
+          cacerts: :public_key.cacerts_get(),
+          depth: 99,
+          server_name_indication: String.to_charlist(System.get_env("SMTP_SERVER")),
+          customize_hostname_check: [
+            match_fun: :public_key.pkix_verify_hostname_match_fun(:https)
+          ]
+        ] ++ tls_opts,
       retries: 1
   end
 end