Accept env variables to override default passwords

Preserves the default passwords corresponding to the OpenStack Install
and Deploy Manual while allowing environment variables to specify user
provided passwords. As well, a single common password for all the service
users can be specified using the SERVICE_PASSWORD environment variable.

This fix restores compatibility with the version of this script released
in Folsom where the environment variables could override default passwords.
It also preserves compatibility with the fix for bug 1073291 that brought
the script into line with the docs (but unfortunately broke backwards
compatibility).

There is no warning emited if the user does not override the default
passwords as was present in Folsom, but not in Grizzly. This makes the
fix output compatible with Grizzly only. Note that the logic for warnings
pre-Grizzly was unclear. The script is, however, clearly named "sample"
and warnings are present in the comments.

Change-Id: I927fcddf04dcb87e5c9252f0874939b17f3c4809
Fixes: bug #1166182