Support Required Args

Problem

In bbctl we are manually making args required, usually like this:

	repoPath := viper.GetString("big-bang-repo")
	if repoPath == "" {
		factory.GetLoggingClient().Error("Big bang repository location not defined (\"big-bang-repo\")")
	}

This is very redundant and fragile and should be consolidated as mentioned in this thread.

Proposal

We should implement another client in the factory that provides func (*ArgClient) GetStringArg(name string, required bool) (string, error) in addition to a similar receiver function for each type we might need, e.g. bool, int, float, etc.