Until now, running `db:dev_seed` created development data for the
default tenant but it was impossible to create this data for other
tenants.
Now the tenant can be provided as a parameter.
Note that, in order to be able to execute this task twice while running
the tests, we need to use `load` instead of `require_relative`, since
`require_relative` doesn't load the file again if it's already loaded.
Also note that having two optional parameters in a rake task results in
a cumbersome syntax to execute it. To avoid this, we're simply removing
the `print_log` parameter, which was used mainly for the test
environment. Now we use a different logic to get the same result.
From now on it won't be possible to pass the option to avoid the log in
the development environment. I don't know a developer who's ever used
this option, though, and it can always be done using `> /dev/null`.
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