Add "security group rule show" command

Add the "os security group rule show" command which will use
the SDK when neutron is enabled, and use the nova client when
nova network is enabled.

Change-Id: I41efaa4468ec15e4e86d74144cc72edc25a29024
Partial-Bug: #1519512
Implements: blueprint neutron-client

doc/source/command-objects/security-group-rule.rst

@@ -67,3 +67,18 @@ List security group rules
 .. describe:: <group>
 
     List all rules in this security group (name or ID)
+
+security group rule show
+------------------------
+
+Display security group rule details
+
+.. program:: security group rule show
+.. code:: bash
+
+    os security group rule show
+        <rule>
+
+.. describe:: <rule>
+
+    Security group rule to display (ID only)