cmd/bosun: add complete influx cred config, docs: update docs to reflect new config

[nathanielc]

Adds configuration so that you can connect to InfluxDB with TLS and/or a username and password.

[gbrayut]

LGTM.

Compiled an ran it locally with no changes to bosun/influxdb settings and it still works as expected. Changed /etc/opt/influxdb/influxdb.conf to use auth-enabled = true under the http section, used /opt/influxdb/influx to create a user for bosun, and added the following settings to the bosun config file

[root@gbcentos7 influxdb]# head /home/gbray/go/src/bosun.org/cmd/bosun/gbcentos7.conf
tsdbHost = gbcentos7:4242
influxHost = gbcentos7:8086
influxUsername = bosun
influxPassword = bosun
graphiteHost = gbcentos7:81
smtpHost = gbcentos7:25
emailFrom = bosun@localhost
httpListen = :80
timeAndDate = 202,75,179,136
stateFile = /home/gbray/go/src/bosun.org/cmd/bosun.state

After restarting bosun and influxdb the influx("opentsdb",'''SELECT value from "os.mem.percent_free" group by host''',"5m","") expression still works as expected.