[nri-config-generator] cli, env var support

[paologallinaharbur]

The support for both cli and env var is custom and possibly fragile, we could:

• move it to Viper (config file is already managed with such library)
• use integration SDK (it would be more "standard" with respect to existing integrations)

[paologallinaharbur]

This task is referring to the env management of the configurator itself, not of the integrations.

Example of code not having a standard management of environment variables:

[workato-integration]

https://issues.newrelic.com/browse/NEWRELIC-4716