feat(env): allow env vars to configure http requests

Implements nginx environment for pact-foundation/pact_broker#193

Author: mefellows

Diff for: container/etc/nginx/main.d/pactbroker-env.conf

@@ -7,3 +7,6 @@ env PACT_BROKER_DATABASE_PORT;
 env PACT_BROKER_BASIC_AUTH_USERNAME;
 env PACT_BROKER_BASIC_AUTH_PASSWORD;
 env PACT_BROKER_LOG_LEVEL;
+env HTTP_PROXY;
+env HTTPS_PROXY;
+env NO_PROXY;