provide API to output the HOT template from Tosca template

[yong sheng gong]

Problem:
   we provide Tosca template for VNFD, VNFFGD, in future the NSD, It is not easy to see
if the template HEAT parser's output for them.

expectation:
   provides an API for users to get HOT template for their TOSCA templates.

   tacker get_hot <vnfd_id>
   tacker get_hot_from_tosca_file <tosca_file>

   these output should be the same to what the VNFM's openstack driver gives to HEAT.

[Sridhar Ramaswamy (srics-r)]

This will make Tacker APIs (openstack) VIM specific. Tacker should be VIM agnostic and look towards supporting non-OpenStack VIMs.