Update code according to Idan's review

- add missing licence headers
- index based string formatting
CFY-1659 Port plugin to Cloudify 3.1

- import ctx instead of having it as an operation argument
- change ctx.properties to ctx.node.properties
- update cloudify-plugins-common dependency.  Since this version is
  not yet in PyPI, we had to specify github source locations.

Signed-off-by: Michał Soczewka <michal.soczewka@codilime.com>