Add code to organize a registry of Radio Units and handle that registry generally everywhere.

RU registry is still populated from cell_list and in practice there still can
be radio units only of the same type, but besides slaplte.load_ru_and_cell
which is aware of that, the rest of the code tries to do everything as if
RUs of different type could be present simultaneously.

Now it is not only lopcomm who can do multiCELL, but also sunwave and SDR as well.

Later we will change the loading to load RU descriptions from shared instances
and they won't be constrained to be Radio Units of the same type. But we need
to prepare a lot to be able to do that.

One more step forward towards MultiRU.

Tests will be added later as full tests for generic MultiRU.

--------

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

```
$ ./pythonwitheggs slapos-render-config.py && git diff -w --no-index config/{old,out}
```

```diff
diff --git a/config/old/enb.cfg b/config/out/enb.cfg
index 884483b0a..cafdf42be 100644
--- a/config/old/enb.cfg
+++ b/config/out/enb.cfg
@@ -1,24 +1,22 @@

-
 {
   log_options: "all.level=error,all.max_size=0,nas.level=debug,nas.max_size=1,s1ap.level=debug,s1ap.max_size=1,x2ap.level=debug,x2ap.max_size=1,rrc.level=debug,rrc.max_size=1,ngap.level=debug,ngap.max_size=1,xnap.level=debug,xnap.max_size=1,phy.level=info,file.rotate=1G,file.path=/dev/null",
   log_filename: "log/enb.log",

-
+  // Radio Units
   rf_driver: {
+      // default-RU 2T2R  (ors)
       name: "sdr",
       args: "dev0=/dev/sdr0",
-
       rx_antenna:"tx_rx",
       tdd_tx_mod: 1,
   },
-  tx_gain: 62,
-  rx_gain: 43,
-
+  tx_gain: [62, 62],
+  rx_gain: [43, 43],
   com_addr: "127.0.1.2:9001",
   // LTE core network
   mme_list: [
@@ -36,6 +34,8 @@

   // LTE cells
   cell_list: [
+
+    // default  (default-RU)
     {
       rf_port:      0,
       n_antenna_dl: 2,
diff --git a/config/old/gnb.cfg b/config/out/gnb.cfg
index fd57ca3dc..7818b4ea8 100644
--- a/config/old/gnb.cfg
+++ b/config/out/gnb.cfg
@@ -1,24 +1,22 @@

-
 {
   log_options: "all.level=error,all.max_size=0,nas.level=debug,nas.max_size=1,s1ap.level=debug,s1ap.max_size=1,x2ap.level=debug,x2ap.max_size=1,rrc.level=debug,rrc.max_size=1,ngap.level=debug,ngap.max_size=1,xnap.level=debug,xnap.max_size=1,phy.level=info,file.rotate=1G,file.path=/dev/null",
   log_filename: "log/enb.log",

-
+  // Radio Units
   rf_driver: {
+      // default-RU 2T2R  (ors)
       name: "sdr",
       args: "dev0=/dev/sdr0",
-
       rx_antenna:"tx_rx",
       tdd_tx_mod: 1,
   },
-  tx_gain: 62,
-  rx_gain: 43,
-
+  tx_gain: [62, 62],
+  rx_gain: [43, 43],
   com_addr: "127.0.1.2:9001",
   // NR core network
   amf_list: [
```