HOT templates get_attr allows extra attributes

The HOT templates now allows the use of extra attributes for the
get_attr function. Wich this change is possible to traverse complex
resource attributes.

For example, getting an IP address for a server on a given network:

when the network name is stable, and always known:
get_attr:
- my_server
- networks
- public
- 0

or if the network name is something other than public:
get_attr:
- my_server
- networks
- get_param: my_network_name
- 0

If one of the extra arguments does not match with a key or a valid
index, an empty string is returned.

As a bonus track the Fn::GetAttr support was removed from the HOT
templates.

Change-Id: Ia7361fbd10611cc796bd90e781f4ae387c970dbd
Implements: blueprint hot-select
Merge "HOT templates get_attr allows extra attributes"