When trying to validate a template that should later be filled in with
`pungi-config-dump`, there will be errors about undefined variables.
These are meant to be set when the template is populated.
This patch adds support for `-e`, `--define` argument to the validation
script that can be used to suppress these errors.
Alternatively a JSON file is read from the directory with config file
that can contain values for the variables.
The `--define` option is changed in both validation and dumping to allow
empty string as an accepted value.
JIRA: COMPOSE-3599
Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>
