Introduce support for AmarisoftEPC

* A new abstract generic base class EPC is created
* srsEPC and AmarisoftEPC inherit from that class
* options are loaded from defaults.conf in cascade. First generic "epc",
  afterwards the specific enb type.
* A new scenario is added to select the EPC type to use. srsEPC is the
  default unless stated by an scenario.
* AmarisoftEPC delegates setup of the tun IP address to an "ifup"
  script. As a result, since we run without root permissions (ony with
  CAP_NET_ADMIN), the ifup script itself is unablet o set the IP
  address. To solve this, we introduce a new osmo-gsm-tester helper
  script which must be installed in the slave node which can be called
  through sudo to increase privileges to do so.

With this commit, I can already get srsUE<->amarisoftENB<->amarisoftEPC
to pass ping and iperf3 4g tests.

Change-Id: Ia50ea6a74b63b2d688c8d683aea11416ad40a6d3
diff --git a/utils/sudoers.d/osmo-gsm-tester_amarisoft_ltemme_ifup b/utils/sudoers.d/osmo-gsm-tester_amarisoft_ltemme_ifup
new file mode 100644
index 0000000..a8cd298
--- /dev/null
+++ b/utils/sudoers.d/osmo-gsm-tester_amarisoft_ltemme_ifup
@@ -0,0 +1 @@
+%osmo-gsm-tester ALL=(root) NOPASSWD: /usr/local/bin/osmo-gsm-tester_amarisoft_ltemme_ifup.sh