We currently have LTE-specific handover configuration in under cell_list and
NR-specific handover configuration in under nr_cell_list. Those configuration
are different.

However in upcoming MultiRU we will need to handle LTE->NR handover, NR->LTE
handover, and also Intra-ENB handover in addition to Inter-ENB handover we
currently do.

-> Move handover code into common function as a preparatory step for that.

In the future the handover code for LTE and NR cells will be the same, so it
makes sense to move that code to common place to avoid duplication.

For rendered enb.cfg this unification introduces only space and trivial
changes as shown in the appendix.

/cc @jhuge, @lu.xu, @tomo, @xavier_thompson, @Daetalus
/proposed-for-review-on nexedi/slapos!1528
/reviewed-by TrustMe

Appendix. Diff for rendered enb.cfg and gnb.cfg before and after this patch

```
$ git diff -w --no-index config/{old,out}
```

```diff
diff --git a/config/old/enb.cfg b/config/out/enb.cfg
index 43301ee13..9dcca16c7 100644
--- a/config/old/enb.cfg
+++ b/config/out/enb.cfg
@@ -45,16 +45,18 @@
       root_sequence_index: 204,
       dl_earfcn: 36100,
       inactivity_timer: 10000,
+
       // Handover
       ncell_list: [
         // Inter-ENB HO
         {
+          rat: "eutra",
           n_id_cell: 35,
           dl_earfcn: 700,
           cell_id: 0x12345,
           tac: 123,
-        }],
-
+        },
+      ],

       // Carrier Aggregation
       scell_list: [
diff --git a/config/old/gnb.cfg b/config/out/gnb.cfg
index 2127a2f6b..23b07d6e1 100644
--- a/config/old/gnb.cfg
+++ b/config/out/gnb.cfg
@@ -57,6 +57,7 @@
     ssb_pos_bitmap: "10000000",

     inactivity_timer: 10000,
+
       // Handover
       ncell_list: [
         // Inter-ENB HO
@@ -74,8 +75,8 @@
           ssb_period: 20,
           ssb_offset: 0,
           ssb_duration: 1,
-      }],
-
+        },
+      ],

     // tune NR parameters for the cell
     manual_ref_signal_power: true,
```