Allow to configure WEBAPP_DEFAULT_URI environment variable

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.
• Tajine Software allows to use a bin/console command that generates URLs.
• For the moment, PKI Adullact does not use, in production, 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/'

see also : https://gitlab.adullact.net/search?search=WEBAPP_DEFAULT_URI&project_id=1156&group_id=5636&search_code=true&repository_ref=main

similar to :

• Allow to configure WEBAPP_DEFAULT_URI environme... (Comptoir/puppet-comptoir-du-libre#3 - closed)
• Comptoir/puppet-comptoir-du-libre!4 (merged)!