fixes #11473 - remove re-init of domain select2 on env selection

After an environment was selected, the reloadOnAjaxComplete function
re-initialised every select2, but other dropdowns triggered
onContentLoad which excluded those with the without_select class.

Change reloadOnAjaxComplete to be consistent and also explicitly
re-init the subnet dropdown when the domain dropdown causes it to
change.

Also destroy select2 fields before storing the interface in the
"hidden" storage, allowing it to be re-initialised when the modal
is displayed again.

(cherry picked from commit 501853b511d52f078c1d0933afec955b25356e54)