heat engine : reinstate resources import

Reinstate the resources import removed by 06f4f85, without this
we fail to create any resources (GenericResource is created for
all resources due to no resource plugins being imported).

bug 1133434

Change-Id: I2b24d58dee83a426deee09f8a9a121f503534de4
heat engine : fix variable/import shadowing in service.py

The resources import is shadowed by a variable declaration in
validate template, which is confusing, so add a prefix to
differentiate the variable from the import

bug 1133434

Change-Id: Ia7c451c3a2588ddff114da9897b3bcf58e68cf0c