Support ENV variables for user, password and db

[yanfali]

Adding the following environmental variables to allow configuration of username, password and database to make it less trivial to get this information from top:

• INFLUXDB_PROXY_PASSWORD
• INFLUXDB_PROXY_USERNAME
• INFLUXDB_PROXY_DATABASE

While these values are still available from /proc/<processid>/environ on a linux based system you have to have elevated privileges to look at them. With the current variables you can simply look at top to see the username, password and database.

[yanfali]

BTW, I deliberately exposed these values in the help text so it's easy to verify that the values have been passed in from os.Env. e.g.

INFLUXDB_PROXY_DATABASE=influx1 INFLUXDB_PROXY_USERNAME='banana' INFLUXDB_PROXY_PASSWORD='banacheck' influxdb-collectd-proxy --help

[hoonmin]

Sure! Merged your PR. Thanks!

[yanfali]

Thanks @hoonmin