The client now is created without making a call to nginx.
When Collect() is called we try to get the metrics from nginx and if
it's not available it will set 'up' to 0 and not fail anymore.
Closes: https://github.com/nginxinc/nginx-prometheus-exporter/issues/36
* Add nginx.retries cli flag
* Add nginx.retry-interval cli flag
* Add handling of SIGTERM signals
* Add started log message
* Add createClientWithRetries test
* Add time.Duration env variables: timeout, nginxRetryInterval
* Fix error context: On OSS Failed to create client would be appended twice from nginx client and exporter
* Add environment variables to --help flag and readme.md
