Utilize rails autoload to avoid 'already initialized constant' warnings
Utilize rails autoload to avoid 'already initialized constant' warnings

PglogicalSubscription was using autoload via:

```
include MiqPglogical::ConnectionHandling
```

This rake task was doing require.  Since this rake task loads rails environment,
we can utilize autoload here too in order to avoid reading miq_pglogical twice:
once from autoload and once from require.

To recreate this, db:rollback db:migrate, was showing this:

```
joerafaniello@Joes-MBP-2 manageiq % be rake db:rollback db:migrate
** ManageIQ master, codename: Morphy
== 20210625185428 FixVpcFlavorsSti: reverting =================================
-- Resetting IBM Cloud VPC Flavors STI class
   -> 0.0145s
   -> 0 rows
== 20210625185428 FixVpcFlavorsSti: reverted (0.0145s) ========================

== 20210625185428 FixVpcFlavorsSti: migrating =================================
-- Fixing IBM Cloud VPC Flavors STI class
   -> 0.0190s
   -> 0 rows
== 20210625185428 FixVpcFlavorsSti: migrated (0.0191s) ========================

/Users/joerafaniello/Code/manageiq/lib/miq_pglogical.rb:8: warning: already initialized constant MiqPglogical::PUBLICATION_NAME
/Users/joerafaniello/Code/manageiq/lib/miq_pglogical.rb:8: warning: previous definition of PUBLICATION_NAME was here
/Users/joerafaniello/Code/manageiq/lib/miq_pglogical.rb:9: warning: already initialized constant MiqPglogical::ALWAYS_EXCLUDED_TABLES
/Users/joerafaniello/Code/manageiq/lib/miq_pglogical.rb:9: warning: previous definition of ALWAYS_EXCLUDED_TABLES was here
```

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