Remove the usage of read_password from library files

The read_password function is defined inside stack.sh
and it cannot be used inside the "public library interface"
provided by DevStack.

Move the calls found inside library files to stack.sh,
following the same pattern of the other calls to read_password.

Change-Id: I8adc6723b677dfac2bef735f660e056c498bf773
diff --git a/stack.sh b/stack.sh
index 022d5b9..8d4042d 100755
--- a/stack.sh
+++ b/stack.sh
@@ -691,7 +691,14 @@
 # The available database backends are listed in ``DATABASE_BACKENDS`` after
 # ``lib/database`` is sourced. ``mysql`` is the default.
 
-initialize_database_backends && echo "Using $DATABASE_TYPE database backend" || echo "No database enabled"
+if initialize_database_backends; then
+    echo "Using $DATABASE_TYPE database backend"
+    # Last chance for the database password. This must be handled here
+    # because read_password is not a library function.
+    read_password DATABASE_PASSWORD "ENTER A PASSWORD TO USE FOR THE DATABASE."
+else
+    echo "No database enabled"
+fi
 
 
 # Queue Configuration
@@ -729,6 +736,16 @@
 fi
 
 
+# Nova
+# -----
+
+if is_service_enabled nova && [[ "$VIRT_DRIVER" == 'xenserver' ]]; then
+    # Look for the backend password here because read_password
+    # is not a library function.
+    read_password XENAPI_PASSWORD "ENTER A PASSWORD TO USE FOR XEN."
+fi
+
+
 # Swift
 # -----