Allow ClickHouse migrations to be skipped

What does this MR do and why?

This MR fixes Unable to skip ClickHouse migrations (#581777) where configured ClickHouse YML without an existing ClickHouse DB fails the gitlab:db:configure task.

The fix is an optional environment variable: SKIP_CLICKHOUSE_MIGRATIONS.

Running the rake task with the env variable will not invoke ClickHouse migrations:

SKIP_CLICKHOUSE_MIGRATIONS=true bundle exec rake gitlab:db:configure

References

Unable to skip ClickHouse migrations (#581777)

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Related to #581777