fix schema migration hooks (partial) SQL Server requires some additional steps to alter columns (index drop and restore, etc...) and this in turn requires passing a connection on the DB to find out the indexes. Also partially fix an issue where the null_allowed value was not correctly computed. Still some problems in there (see https://www.logilab.net/cwo/ticket/1109558)

# copyright %(year)s %(author)s, all rights reserved.
# contact %(author-web-site)s -- mailto:%(author-email)s
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"""%(distname)s precreate script, executed at instance creation time or when
the cube is added to an existing instance, before the schema is serialized.

This is typically to create groups referenced by the cube'schema.
"""