Ignite CLI: Allow to pass multiple files with cluster configuration for init command

Motivation:

It is nice to have possibility to pass multiple files with cluster configuration on init step. The main idea of configuration separation is to create a complex configuration based on simple ones, for example in tests. Another reason for that can be restricted access to part of configuration that contains some credentials.

What to to:

• Rename "--config-file" option to "config-files"

• Allow to pass multiple "--config-files" option for cluster init command in cli tool.

• Merge content of passed files with configuration based they order.