Enhancing BluePrintJinjaTemplateService
* Using CustomJinjavaInterpreter to override getAsString API, and returning context.currentNode.master.image instead of empty string
* Earlier rendered result was not keeping unresolved variable rather replacing with empty string, and above helps keeping those.
* Upgrading jinja.version to 2.7.0 from 2.5.1 and adding related Junit Test
Issue-ID: CCSDK-3895
Signed-off-by: Singal, Kapil (ks220y) <ks220y@att.com>
Change-Id: I00da64811883957f44f819a44405e073ed7a7755