Move SchemaMigration model from ManageIQ to ManageIQ::Schema plugin

[bdunne]

Fixes ManageIQ/manageiq#16670

[miq-bot]

Checked commit bdunne@fec46b1 with ruby 2.3.3, rubocop 0.52.0, haml-lint 0.20.0, and yamllint 1.10.0
1 file checked, 3 offenses detected

app/models/schema_migration.rb

• ❗ - Line 11, Col 5 - Style/MultilineIfModifier - Favor a normal unless-statement over a modifier clause in a multiline statement.
• ❗ - Line 13, Col 5 - Style/MultilineIfModifier - Favor a normal unless-statement over a modifier clause in a multiline statement.
• ❗ - Line 7, Col 5 - Style/RescueStandardError - Avoid rescuing without specifying an error class.

[bdunne]

@Fryguy Please review

[simaishi]

Gaprindashvili backport details:

$ git log -1
commit 45dcef58852ebf062addd4d1b0908da8a5f16f55
Author: Jason Frey <fryguy9@gmail.com>
Date:   Fri Mar 2 15:52:47 2018 -0500

    Merge pull request #175 from bdunne/schema_migration
    
    Move SchemaMigration model from ManageIQ to ManageIQ::Schema plugin
    (cherry picked from commit bf42ada3662b64281b3b89b86ca5e6afc809def9)
    
    https://bugzilla.redhat.com/show_bug.cgi?id=1553397