iscsi: don't set default value for trusted_ip_list

iscsi: don't set default value for trusted_ip_list

It restricts access to the iSCSI API.
It can be left empty if the API isn't going to be access from outside the
gateway node

Even though this seems to be a limited use case, it's better to leave it
empty by default than having a meaningless default value.

We could make this variable mandatory but that would be a breaking
change. Let's just add a logic in the template in order to set this
variable in the configuration file only if it was specified by users.

Closes: https://bugzilla.redhat.com/show_bug.cgi?id=1994930
Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>
Co-authored-by: Dimitri Savineau <dsavinea@redhat.com>