Adapt net interface delete to 'rockstor' service null config #2819

[phillxnet]

Block non-rocknet interface deletions for the default/no management/Web-UI interface configuration. Raising a user visible exception explaining the interface delete rejection, and advising the need to configure the 'rockstor' service first. This way, for default/no 'rockstor'/Web-UI service configurations (no management interface defined) we avoid inadvertent Web-UI outage. Once a non default 'rockstor' service/Web-UI Network interface is established, only that interface will be bared from delete.

Fixes Web-UI silent but logged failure to properly process/fence an interface delete request with default 'rockstor'/Web-UI configuration.

Fixes #2819

[phillxnet]

Testing

See issue for pre patch behaviour: a silent failure in our fencing to guard against inadvertent removal of the Web-UI associated 'management' interface; but only when a management interface has yet to be defined: i.e. default 'rockstor' service configuration.

Resulting in Web-UI inaccessibility and the requirement to re-connect that interface via, for example, nmcli dev connet eth0.

After the proposed patch, for a default configuration of the 'rockstor'/Web-UI network interface, we disable/block all non rocknet interface deletions with the following Web-UI/user-visible exception:

On the grounds that we have no configuration to advise us otherwise.

Once a non-default 'rockstor' service config has been established: i.e. an interface for the Web-UI has been selected, this interface only is excluded from all deletion requests.

Noting that we can, with a Web-UI config, now delete other non-Web-UI associated interfaces as expected.