add an option to specify configuration file path manually

[outscale-mgo]

problem is that I can't properly test that osc-cli work in brew formula:
brew create a temporary test directory for tmp files, but doesn't sandbox the whole filesystem,
so creating a file in $HOME is a no go, as I can't modify user filesystem when testing a package.
So that mean I need to use brew tmp directory when testing osc-cli in homebrew.

Also, most program that use conf file generally have a way to specify config file path in they argument, so I guess that's what we should do.