credential: stop using deprecated property for email

In 44e0b22f (Move to new API, detailed properties take precedence,
2011-09-09) we introduce a new data model for coordinates, in the case
of emails, we have "url_string" fro the "detailed" form and
"coordinate_text", for the "store what user entered" form.

In some places of erp5_credential, we use setDefaultEmailText, which
calls Coordinate.setText which uses the deprecated Coordinate.fromText,
which sets the same value for "url_string" and "coordinate_text", which
seems a questionnable behavior, because the data is saved twice and
some code might be using the wrong property.

This changes every usage to the new coordinate_text property