cyclades: Limit the number of Volumes metadata

* Add a setting in 20-snf-cyclades-app-api.conf to limit the number of
metadata items for volumes. The default value is set to 10.
* Add checks in volume create and update functions to enforce the above
setting.
cyclades: Limit the number of VM metadata items

* Add a setting in 20-snf-cyclades-app-api.conf to limit the number of
metadata items for Virtual Machines. The default value is set to 10.
* Add checks in VM create and update functions to enforce the above
setting.
docs: Update blockstorage api guide

Add a section explaining how to update Volume Metadata
ci: Handle an uncaught exception

Properly handle and print an error message if a valid GitHub pull
request isn't given with --pull-request option.