Feature logging verbosity via env in servercmd

[repson]

An environment variable has been added to configure register verbosity in servercmd.

The RegisterFlags, ConfigureFromEnv and parseVerbosity functions will help us to better organise the code and manage the contents of the VERBOSITY environment variable in the servercmd package.

In addition, new int constants have been added to define the default, maximum and minimum verbosity values.

[sevein]

I've just realized that we were actually able to parse root flags. For example, try using:

env CCP_V=10 CCP_DEBUG=true go run . server

So my diagnosis was incorrect, my apologies! I missed the part where we call rootConfig.RegisterFlags(fs) from servercmd.

[sevein]

If you wanted to validate the verbosity we could try something like this: ccp...dev/issue-9-verbosity-validation. You can push to that branch if you want to make further changes. Validate mutates the configuration attribute. The function return an error in case we want future validation to raise errors where we want to prevent the program from running.