Enable the definition of list type variables

As of today Bamboo variables are only single (key, value) pairs. If one wants to define Parametrized Plans, it implies to remember all options in order to run the customized builds. This is quite error prone or leads to the duplication of plans to go around this issue.

Enabling the definition of list type variables would solve this situation.

Features:
#1 When running a customized plan, the user would be presented with the drop down with the list of values for the parameter
#2 The list of values would be available from the REST API (/plan/{projectKey}-{buildKey}?expand=variableContext)
#3 Possibility to define the default value