Fix all missing "os to openstack" transformation

Checked and fix all missing of the review:
https://review.openstack.org/#/c/412669/
https://review.openstack.org/#/c/412673/
https://review.openstack.org/#/c/412678/

Change-Id: Ice85958874121eeb574d0b4cfa58e1590b014ed6

doc/source/command-objects/group.rst

@@ -12,7 +12,7 @@ Add user to group
 .. program:: group add user
 .. code:: bash
 
-    os group add user
+    openstack group add user
         [--group-domain <group-domain>]
         [--user-domain <user-domain>]
         <group>

doc/source/command-objects/server.rst

@@ -12,7 +12,7 @@ Add fixed IP address to server
 .. program:: server add fixed ip
 .. code:: bash
 
-    os server add fixed ip
+    openstack server add fixed ip
         <server>
         <network>
 

doc/source/command-objects/snapshot.rst

@@ -13,7 +13,7 @@ Create new snapshot
 .. program:: snapshot create
 .. code:: bash
 
-    os snapshot create
+    openstack snapshot create
         [--name <name>]
         [--description <description>]
         [--force]

doc/source/command-objects/volume-backup.rst

@@ -12,7 +12,7 @@ Create new volume backup
 .. program:: volume backup create
 .. code:: bash
 
-    os volume backup create
+    openstack volume backup create
         [--container <container>]
         [--name <name>]
         [--description <description>]

doc/source/command-objects/volume-snapshot.rst

@@ -12,7 +12,7 @@ Create new volume snapshot
 .. program:: volume snapshot create
 .. code:: bash
 
-    os volume snapshot create
+    openstack volume snapshot create
         [--volume <volume>]
         [--description <description>]
         [--force]