Align machine_spec with pydoc

Align the name of the machine_spec keys as defined in the pydoc

Change-Id: I3e9c3609ea81b53b77cefcda63bbb4a8b0c6510c
Signed-off-by: Adam Israel <adam.israel@canonical.com>
diff --git a/tests/base.py b/tests/base.py
index 912eab9..663e89a 100644
--- a/tests/base.py
+++ b/tests/base.py
@@ -740,8 +740,8 @@
             )
 
             machine_spec = {
-                'host': hostname,
-                'user': 'ubuntu',
+                'hostname': hostname,
+                'username': 'ubuntu',
             }
 
         await self.n2vc.DeployCharms(