allow custom strings in configuration

See merge request !3

we're not in global context anymore, the local variable is lost when
exiting the function.

The script doesn't read configuration at all, usage with same user as web
server is not mandatory, and there is no point at having two different
usage examples, especially if contradictory.

This allows to define multiple additional configuration-dependant strings for
templates, especially useful in multi-tenants setup.

Master+isolate metadata update

See merge request aai/SWITCHwayf!1

- config files are now in etc subdirectory
- web files are now in www subdirectory
- library files are now in lib subdirectory
- script files are now in bin subdirectory

- regenerate_metadata() function has been merged with update_metadata(),
avoiding duplicated work
- failure to open lock file or to acquire lock results in immediate
function exit