Create NOVA_READY_TIMEOUT in is_nova_ready function

The function was introduced in [0] using a hardcoded timeout of 60
seconds which turns out to be too small on slow machines. Create a new
global variable NOVA_READY_TIMEOUT instead so that users can
override the timeout if necessary.

[0] I32eb59b9d6c225a3e93992be3a3b9f4b251d7189

Co-Authored-By: Mohammed Naser <mnaser@vexxhost.com>
Change-Id: I0cd7f193589a1a0776ae76dc30cecefe7ba9e5db
diff --git a/lib/nova b/lib/nova
index 8691c8e..d89d64e 100644
--- a/lib/nova
+++ b/lib/nova
@@ -1000,7 +1000,7 @@
         # because of the dom0/domU split. Just ignore for now.
         return
     fi
-    wait_for_compute 60
+    wait_for_compute $NOVA_READY_TIMEOUT
 }
 
 function start_nova {