use envvars as default value for sensitive argments.

[jspaleta]

Treat influx addr, username and password as sensitive credentials.

It's preferable to read sensitive information from envvars.
commandline arguments can be read from the process table for ex: ps -aux

Solution in this PR:

1. Use os.Getenv() to retrieve default values from env vars when constructing command flags.
2. Do not mark these as required flags, if not set and env var defaults are used.
3. Manual test for empty string values and error out accordingly.

[nikkictl]

I love how you implemented this with backwards compatibility! Since the help usage changed, we'll need to update the readme with the new flag descriptions. If we expect and encourage users to store the sensitive data as environment variables, we should also state that in the readme, and perhaps provide short instruction on how to do so. Lastly, we'll need a changelog entry in the "Unreleased" section.

This is going to be a great security win. Code-wise, this LGTM 👍

[jspaleta]

requested changes made.
I have locally built version running now using envvars

sensuctl handler info influxdb-particle
=== influxdb-particle
Name:                  influxdb-particle
Type:                  pipe
Timeout:               30
Filters:               
Mutator:               
Execute:               RUN:  /usr/local/bin/sensu-influxdb-handler -d 'particle'
Environment Variables: INFLUXDB_ADDR=http://localhost:8086, INFLUXDB_USER=nunya, INFLUXDB_PASS=business

[nikkictl]

💯