Convert exporter credentials to envvars, round 2

[aegershman]

Continuation of #357

Apologies I've taken a weeks longer than I expected to finish this.

It turns out there's only a handful more jobs which pass creds via CLI rather than envvars or config files-- and most aren't using creds directly at all. I've swapped those out, with the exception of the mongodb_exporter which doesn't appear to support passing creds via envvar, so ¯_(ツ)_/¯ no worries

Please let me know if there's anything you'd like changed, etc.; I've got nice and relaxing time now while over break 👌thanks again

• bosh_exporter done in first PR
• bosh_tsdb_exporter
• collectd_exporter N/A
• consul_exporter N/A
• credhub_exporter
• elasticsearch_exporter N/A
• firehose_exporter done in first PR
• graphite_exporter N/A
• haproxy_exporter N/A
• influxdb_exporter N/A
• kube-state-metrics exporter N/A
• memcached exporter N/A
• mongodb_exporter doesn't appear as though the mongodb exporter supports being passed creds via env?
• mysqld_exporter N/A
• nats_exporter N/A
• postgres_exporter N/A
• rabbitmq_exporter N/A
• redis_exporter
• shield_experort
• stackdriver+exporter N/A
• statsd_exporter N/A
• vault_exporter N/A

[frodenas]

Thanks @aegershman!