Introduce grammar to parse task references

Uses antlr4 to parse the task reference syntax. Also,
adds a missing test case to ensure functionality.

Include 'maven' in the zuul config to ensure it's installed on the host
which fixes the error:

 antlr4_runtime requires mvn as a dependency. Please check your PATH.

Change-Id: I439d813f63639558109f547f0e769d6833d4c680