Clarify API extension description of Nova API

On Nova API, we can see two names for each API extension. The one is
"Name", the other is "Alias". The "Name" is represented with camelcase
and the "Alias" is done with characters and hyphens:

 $ nova list-extensions
 +--------------+-----------------------+------------------+---------+
 | Name         | Summary               | Alias            | Updated |
 +--------------+-----------------------+------------------+---------+
 | AdminActions | Enable admin-only ... | os-admin-actions | ...     |
 | Agents       | Agents support.       | os-agents        | ...     |
 | ConsoleOutput| Console log output... | os-console-output| ...     |
 | Consoles     | Interactive Console.. | os-consoles      | ...     |

In Tempest, we can select API extensions what we test and it should be
based on "Alias", because on Nova side we have been a lot stricter about
format/uniqueness for the alias which is seen as the canonical name.
This patch clarifies it.

Change-Id: I3e753c81182af83e01ad3a1f2eb9782c8abb6f50
diff --git a/etc/tempest.conf.sample b/etc/tempest.conf.sample
index 37d4d53..80f28fb 100644
--- a/etc/tempest.conf.sample
+++ b/etc/tempest.conf.sample
@@ -372,11 +372,13 @@
 #disk_config=true
 
 # A list of enabled compute extensions with a special entry
-# all which indicates every extension is enabled (list value)
+# all which indicates every extension is enabled. Each
+# extension should be specified with alias name (list value)
 #api_extensions=all
 
 # A list of enabled v3 extensions with a special entry all
-# which indicates every extension is enabled (list value)
+# which indicates every extension is enabled. Each extension
+# should be specified with alias name (list value)
 #api_v3_extensions=all
 
 # Does the test environment support changing the admin