Preserve overcloud passwords across iterations

In order to make devetest_overcloud.sh ready to support continuous
deployment, we want to preserve the same tripleo-overcloud-passwords across
runs. Change setup-*-passwords to not raise an error (exit 1) when a
password file already exists.

Change setup-undercloud-passwords to make things uniform.

Change-Id: I7f9e50cf18821fc2b07599033870c5a33ba57931
Merge "Preserve overcloud passwords across iterations"