Save interactive passwords to separate file

The interactive password prompt currently saves to .localrc.auto

However, this is removed when you re-run stack; that is required as it
is how we source the localrc bits of local.conf, and we want the
users' changes to be picked up.

The passwords, however, should remain constant, because everything has
already been setup with them.  So write them to a separate file.  Note
we source before localrc so it can still overwrite them.

Some minor flow-changes too

Change-Id: I9871c8b8c7569626faf552628de69b811ba4dac0
Closes-Bug: #1505872
diff --git a/stackrc b/stackrc
index c7c6313..a164335 100644
--- a/stackrc
+++ b/stackrc
@@ -103,6 +103,11 @@
 # be disabled for automated testing by setting this value to False.
 USE_SCREEN=True
 
+# Passwords generated by interactive devstack runs
+if [[ -r $RC_DIR/.localrc.password ]]; then
+    source $RC_DIR/.localrc.password
+fi
+
 # allow local overrides of env variables, including repo config
 if [[ -f $RC_DIR/localrc ]]; then
     # Old-style user-supplied config