Updated version requirement for websockify

This patch updats the websockify version from 'websockify>=0.5.1,<0.6'
to 'websockify>=0.5.1,<0.7'.
websockify is required by nova. The newer version of websockify
has native support for the python 'logging` module.

blueprint: update-to-latest-websockify

Change-Id: I5030bd6d7f4c2326c9cf7c530f649e734322ab51
Merge "Updated version requirement for websockify"