YAML fixes

There are multiple issues wiht YAML loading.

1. Jinja2 renders None values as a string 'None'.  This is not valid YAML
we need to render None values to '~' or 'null' which is the native YAML
None value.

2. Jinja2 renders dict and lists that contain unicode with
u'foo' values.  This is not value YAML syntax.
Because we are serializing dict and lists into YAML, we
need to encode them as valid YAML.  We can override Jinja2 finalize to
use yaml.dump to dump inline YAML.

We use yaml.safe_dump(elem, default_flow_style=True).replace('\n', '')
to generate valid single-line YAML dict and list values.

But this problem highlights the general difficulties with templating and
loading files.
We could avoid this Python->Jinja2->YAML->Python issue by directly
injecting the list or dict after the YAML is loaded.

I'm not sure of the real utility of these templates.

3. On Python 2 YAML loader is rendering all strings
as unicode.  This does not work for Trex because Trex is broken
and badly coded.  Trex does type checking against str() which
is different for Python 2 and Python 3.
The default YAML loader will return native string types, str() or unicode()
for Python 2 and Python 3 respectively.

The bad Trex codes is in convert_val:

https://github.com/cisco-system-traffic-generator/trex-core/blob/master/scripts/automation/trex_control_plane/stl/trex_stl_lib/trex_stl_packet_builder_scapy.py#L674

def convert_val (val):
    if is_integer(val):
        return val
    if type(val) == str:
        return ipv4_str_to_num (is_valid_ipv4(val))
    raise CTRexPacketBuildException(-11,("init val invalid %s ") % val  );

This code is doing type(val) == str.  This is bad and broken.
We can't fix Trex, so we have to render all strings as native str() types

The bug here was that the Heat template loader template_format.py
was overriding the global YAML loader to always return unicode.
We don't want this global override.

To fix this we have to use local subclasses of the yaml.SafeLoader
class.
But in order to dynamically subclass from CSafeLoader or SafeLoader
we have to use the type() builtin to define a new class at runtime.

Once we have new classes defined, we can safely isolate different
YAML constructors and return unicode or not depending on the case.

To be consistent we implement a new yaml_loader.py module to centralize
all non-Heat template yaml loading to ensure correct uncode/str
conversion

Change-Id: Iebf9cf78fbda390977c390436b0869e7bbf503eb
Signed-off-by: Ross Brattain <ross.b.brattain@intel.com>
Signed-off-by: Deepak S <deepak.s@linux.intel.com>
Signed-off-by: Ross Brattain <ross.b.brattain@intel.com>

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