Enabling env var support for license key and app names

[mmahjoubi]

• This will allow the overriding of both the license key and all the app names, through env vars
• For the license key, we just have to define NEWRELIC_LICENSE_KEY
• For an app name, it's more tricky, since we want to support multiple instances per supported backend system. The format is NEWRELIC_<UPPERCASE_PLUGIN_NAME>_<INSTANCE_INDEX> (e.g. if we want to override the first apache_httpd app name, we would define NEWRELIC_APACHE_HTTPD_APP_NAME_0)