Remove references to 'all-plugin' tox environment

The 'all-plugin' tox environment was deprecated by this patch [1].
Instead of the 'all-plugin' it is recommended to use the 'all' tox
environment.

This patch removes any reference to 'all-plugin' tox environment and
updates the documentation so that the installation steps work with
the 'all' tox venv.

[1] https://review.opendev.org/c/openstack/tempest/+/543974

Related-Bug: 1862240
Change-Id: I0a86d9cf33bff6576a6337967d6b663ab7d9f4a6
diff --git a/roles/run-tempest-26/README.rst b/roles/run-tempest-26/README.rst
index 3643edb..8ff1656 100644
--- a/roles/run-tempest-26/README.rst
+++ b/roles/run-tempest-26/README.rst
@@ -21,7 +21,7 @@
    A regular expression used to select the tests.
 
    It works only when used with some specific tox environments
-   ('all', 'all-plugin'.)
+   ('all', 'all-site-packages')
 
    In the following example only api scenario and third party tests
    will be executed.
@@ -47,7 +47,7 @@
    A regular expression used to skip the tests.
 
    It works only when used with some specific tox environments
-   ('all', 'all-plugin'.)
+   ('all', 'all-site-packages').
 
        ::
            vars:
diff --git a/roles/run-tempest/README.rst b/roles/run-tempest/README.rst
index d9f855a..04db849 100644
--- a/roles/run-tempest/README.rst
+++ b/roles/run-tempest/README.rst
@@ -21,7 +21,7 @@
    A regular expression used to select the tests.
 
    It works only when used with some specific tox environments
-   ('all', 'all-plugin'.)
+   ('all', 'all-site-packages').
 
    In the following example only api scenario and third party tests
    will be executed.
@@ -56,7 +56,7 @@
    A regular expression used to skip the tests.
 
    It works only when used with some specific tox environments
-   ('all', 'all-plugin'.)
+   ('all', 'all-site-packages').
 
        ::
            vars: