Return None when get_attr cannot resolve a value

To prevent validation failures prior to resource creation, get_attr
returns None if the attribute or the sub-value cannot be resolved.
This returns this function to consistency with prior behavior and allows
non-string attribute values to be resolved to their proper default
"empty" data types during initial template validation.

Change-Id: I0e02174c43be65a647358a2047da8213635f85d8
Closes-Bug: 1287397
Merge "Return None when get_attr cannot resolve a value"