`tt` ignore specified config

[dmyger]

tt does not use config file specified with --cfg option.

Steps to reproduce:

mkdir tmp && cd tmp
tt init
cp tt.yaml tt.yml
tt --cfg tt.yml

Got error:

 ⨯ Failed to configure Tarantool CLI: failed to get access to configuration file: more than one YAML files are found:
tt.yaml, tt.yml
Ambiguous selection

The problem is that tt searches for a configuration if one was explicitly specified.