Update ppa installation commands (#302)

* Update ppa installation commands

Installing canonical-certification-client or canonical-certification-server is enough to get the full stack of dependencies.
Moreover the old commands were still referring to the plainbox-prefixed package names.

* Remove mention of checkbox-provider-certification-client in the install steps

checkbox-ng/docs/index.rst

@@ -38,18 +38,18 @@ Checkbox can be installed from a :abbr:`PPA (Personal Package Archive)`.
 .. code-block:: bash
 
     $ sudo add-apt-repository ppa:hardware-certification/public
-    $ sudo apt-get update
-    $ sudo apt-get install checkbox-ng plainbox-provider-resource-generic plainbox-provider-certification-client plainbox-provider-checkbox canonical-certification-client
+    $ sudo apt update
+    $ sudo apt install canonical-certification-client
 
-The ``plainbox-provider-certification-client`` package will pull a lot of
+The ``canonical-certification-client`` package will pull a lot of
 desktop-related packages. If you intend to run tests on a server, run these
 commands instead:
 
 .. code-block:: bash
 
     $ sudo add-apt-repository ppa:hardware-certification/public
-    $ sudo apt-get update
-    $ sudo apt-get install checkbox-ng plainbox-provider-resource-generic plainbox-provider-certification-server plainbox-provider-checkbox canonical-certification-server
+    $ sudo apt update
+    $ sudo apt install canonical-certification-server
 
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