Set a maximum metadata value for Volumes/VMs and minor doc updates

This patch set:
* Introduces two new settings for Cyclades adding maximum metadata values for VMs and Volumes.
* Adds checks to enforce the above settings.
* Updates `Changelog` to reflect the above.
* Adds a new section in `Blockstorage API Guide` on how to update Volume Metadata.
* Properly handle and print an error message if a valid GitHub pull reqeust isn't given with --pull-request option in snf-ci.

Suggested reviewer: @cstavr