By using viper.GetString(flag), viper will return the values of that
argument from a precedence order (including CLI arguments). However, if
a value was passed in through an environment variable or as a value in
the config file, it would skip the validation step since that logic was
defined against the FlagSet for command line arguments.

This change causes validation to be done across all input methods.

Fixes #157

Signed-off-by: Bob Callaway <bcallawa@redhat.com>