Do not subscribe portal_alarms unconditionally.

In case of erp5_core business template update this code is called (with update=True argument)
and thus it will effectively override previous portal_alarms' state which
is wrong as no "general purpose" code should overwrite potential
developer configuration.
Tests passing at https://nexedi.erp5.net/test_result_module/20200324-3BC4D696 thus merging right away.

See merge request !1086