Add support for "targetTag" variable of Docker Build promotion API

[rybrow]

Looking at the docs and the source code for the promote_docker_image method, it doesnt look like the "targetTag" variable is supported as per https://www.jfrog.com/confluence/display/JFROG/Artifactory+REST+API#ArtifactoryRESTAPI-PromoteDockerImage

• https://devopshq.github.io/artifactory/#promote-docker-image
• https://github.com/devopshq/artifactory/blob/master/artifactory.py#L2222

We should be able to specify target tags similar to the JFROG API via this library , for example

path.promote_docker_image("docker-local", "docker-release-candidate", "my-application", "0.5.1", target_tag="0.5.1-RC1")

[beliaev-maksim]

I think this was added only in the latest version of API