Add MongoDB 4.2 to linux images (#591)

* Add MongoDB 4.2 to linux images

* use the same installation script for all versions

* Add helpers to get Ubuntu version

* Improve helper functions

* Rename install.sh to os.sh

Co-authored-by: Sergey Dolin <v-sedoli@micorosoft.com>

images/linux/scripts/helpers/os.sh

@@ -0,0 +1,20 @@
+#!/bin/bash
+################################################################################
+##  File:  install-helpers.sh
+##  Desc:  Helper functions for installing tools
+################################################################################
+
+function isUbuntu16
+{
+    lsb_release -d|grep 'Ubuntu 16' > /dev/null
+}
+
+function isUbuntu18
+{
+    lsb_release -d|grep 'Ubuntu 18' > /dev/null
+}
+
+function getOSVersionLabel
+{
+    isUbuntu16 && echo xenial || echo bionic
+}