Stop coercing provider res. attributes to strings

Attributes of a resource defined in another template were
automatically converted to a string. If a provider resource returned
{"ip": ..., "name": ...} as an attribute value, it was converted to
"{'ip': ..., 'name': ...}".

This fix makes the behaviour consistent with the way stack outputs and
native resource work (i.e. can return strings, lists and maps).

Closes-Bug: #1297938
Change-Id: I7f27910e0ec877738998ff80c44e12d3bd4bfa7b
Merge "Stop coercing provider res. attributes to strings"