importMetadata should be configurable

Summary

Every install that uses Renater or some other large federation needs to do some fine-tuning on certain SP/IDPs

Our doc currently states:

For Renater, you need to customize some settings of the script, copy it and edit configuration

Instead, the script should use a configuration file to customize services

Design proposition

Example:

# Default exported attributes for IDPs
[exportedAttributes]
cn=0;cn
eduPersonPrincipalName=0;eduPersonPrincipalName
...

# Default options for SPs
[defaultSpOptions]
CheckSLOMessageSignature=1
...

# Specific SP config
[sp:https://test-sp.federation.renater.fr]
attribute_required=0
attribute_required_uid=1
NameIDFormat=persistent