Implement get_input intrinsic function

Implement the intrinsic function get_property in the TOSCA template:
Example:
    github_url: { get_property: [ SELF, github_url ] }
Implement by traversing the TOSCA graph and computing the value needed.
This needs to be done in the base class HotResource for the general case, which would be translated to a get_param function in HOT.
In special case, this to be overridden in the specific TOSCA translation class. For instance, in the ToscaCompute class for the TOSCA type:
      tosca.nodes.Compute
the get_property function for CPU does not get translated to an equivalent HOT function because one does not exist. So in this case, the general translation in the base HotResource class does not work. Rather, the ToscaCompute class will need its own translation to compute the correct flavor using the CPU value along with other parameters.