osmo-bts-trx: Big refactor and cleanup of osmo-trx related code

* This commit is a preparation for future commits to add support for
different osmo-trx devices and backends like osmo-trx-lms.

* Drop deprecated osmo-trx-* cmd line params and use VTY cfg to set them.

* As number of osmo-trx related osmo-gsm-tester attributes grow, group
them togther in an "osmo_trx" dictionary.

Change-Id: I77d29413c9e3b600b796627ba366f80c3281b7e1
diff --git a/example/defaults.conf b/example/defaults.conf
index 5bdf257..fc4c324 100644
--- a/example/defaults.conf
+++ b/example/defaults.conf
@@ -75,3 +75,8 @@
     - {}
     - phys_chan_config: TCH/F # nanobts only supports PDCH in TRX0.
     - phys_chan_config: TCH/F
+
+osmo_trx:
+  type: uhd
+  launch_trx: true
+  clock_reference: internal