Resolve "Allow to configure WEBAPP_DEFAULT_URI environment variable"

Closes #3 (closed)

Context:

• value of WEBAPP_DEFAULT_URI will be visible in /admin/configuration.
• It is mandatory to configure WEBAPP_DEFAULT_URI, if the bin/console commands are to generate URLs.
• For the moment, Comptoir Software does not use a bin/console command that generates URLs.
• This Puppet module does not currently allow this environment variable to be configured.

# Webapp - Default URI used to generate URLs in a non-HTTP context
WEBAPP_DEFAULT_URI='https://example.org/'