MXS-1782: Take getCollectionAsResource into use

Each `show` type command that takes a resource name now also has a version
that prints all resources of that type.

Added test cases for newly added commands.

maxctrl/test/diagnostics.js

@@ -10,6 +10,14 @@ var tests = [
     'list modules',
     'list users',
     'list commands',
+    'show servers',
+    'show services',
+    'show monitors',
+    'show sessions',
+    'show filters',
+    'show modules',
+    'show maxscale',
+    'show logging',
     'show server server1',
     'show service RW-Split-Router',
     'show monitor MariaDB-Monitor',