Add support for DSM version 7.2

[Kieren]

In DSM 7.2 the Docker package name changes again, so added new SYNO_DOCKER_SERV_NAME7_2 variable to track. Refactored execute_wait_for_docker() to seperate server name command construction logic and remove code redundancy.

[vjm]

this is also working for me -- i came to make my own pull request and saw this already here! great work!