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 #18199 - empty parameter value is an empty string, not nil

Refs #18143 - extend snippet_if_exists too

fixes #18159 - change incorrect clearing of test request session

fixes #18155 - add string to empty form-encoded arrays in test

This test attempts to send an empty `:smart_proxy_ids => []` parameter
emulating the UI to unset smart_proxy_ids on a taxonomy, but Rails 5
filters this empty parameter out when converting it internally to a form...

fixes #17545 - adds UI notification support

- Initial data model for notification support, based on manageiq design.
- addes notification JSON endpoints.

Fixes #18143 - enable variables for snippets (#4205)

Fixes #18045 - Puppet classes show up choosing only env.

After #3551 was merged, the hosts controller requires both environment
and hostgroup to be set in order to display puppetclasses.

It shouldn't be required to have both, so we should check what's
available and use it. If it's only the hostgroup or the environment, it...

Fixes #6502 - List ignored classes and environments on import

When importing environments and classes from puppet, ignored
environments and classes defined in ignored_environments.yml
aren't aparent.

This will add them to the list to import as ignored and...

Fixes #17653 - Allow all parameters api to unhide values