Fix issue with -g option

[filoboss]

Description
When performing a spark migrate with the -g option to use a different DBgroup other than the default, and when there's no default database configured, the migration fails. This is because $this->ensureTable() is executed before $this->setGroup($group). I've simply moved $this->ensureTable() to execute after setting the group.

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide