fixes #18176 - replace clear_association_cache with reload

Reload is still necessary in Rails 5 as interfaces association will be
cached. The BMC test change is necessary to reload smart proxies
associated to the subnet after their features were altered - previously...

fixes #16792 - don't modify AR record attributes in-place

Rails 5 may call #freeze on strings passed in as an attribute during
validation, so attributes must not be edited in-place with gsub! and
similar methods.

fixes #16725 - add plugin extension point for smart proxies

fixes #16557 - move tests into modern Rails layout

Models, controllers, mailers and helpers are now in test/ subdirs
matching app/ subdirs. Service, miscellaneous and old test/lib/ tests
are now in test/unit/ separated from models, which obsoletes the custom...