Network testing: ansible playbook to build a node with reduced block creation time

[gleb-urvanov]

Create ansible playbook to build a node with reduced block creation time

[mnaamani]

Done in #1225

[mnaamani]

Its done by modifying the source code by running a script: https://github.com/Joystream/joystream/blob/master/scripts/alter-block-creation-time.sh

Not the ideal way. In future we want to make some of the runtime constants configurable at build time somehow.