Use password masking utility provided in Oslo

Implement the mask_password function provided in Oslo-incubator's
log.py. Instead of having a specific version in Nova different from
other projects that are essentially doing the same thing.

Change-Id: I7e04b7d31d4d6959b17b1da9654553042eec70f1
Closes-Bug: #1247217
Merge "Use password masking utility provided in Oslo"