This commit introduces an additional (implicit) option that signals if
the E2 agent is to be activated or not. When reading the configuration,
we now check if all fields are set, and activate the E2 agent only if
this is the case. If not, we only print a warning and start the gNB
without the agent.

Note that previously, if not all fields are set correctly, the gNB
stopped.

To implement this behavior, it is necessary to change the defaults of
the RIC IP and the directory for SMs to a (non-NULL) default value.
Otherwise, the configuration system would not consider the field to be
set to the default, and wrongly assumes the field is set, even though it
is not; at least, this change is necessary to make config_isparamset()
return NULL if the configuration is not set.