Build System: Enable python3 compatibility.

* Consistently use spaces in rest-api-templates/asterisk_processor.py.
* Exclude third-party from docs/full-en_US.xml.
* Add docs/full-en_US.xml to .gitignore.
* Use list() to convert python3 view.
* Use python3 print function.
* Replace cmp() with equivalent equation.
* Replace reference to out of scope subtype variable with name
  parameter.
* Use unescaping triple bracket notation in mustache templates where
  needed.  This causes behavior of Python2 to be maintained when using
  Python3.
* Fix references to has_websocket / is_websocket in
  res_ari_resource.c.mustache.
* Update calculation of has_websocket to use any().
* Use unicode mode for writing output file in transform.py.
* Replace 'from swagger_model import *' with explicit import of required
  symbols.

I have not tested spandspflow2pcap.py or voicemailpwcheck.py, only the
print syntax has been fixed.

Change-Id: If5c5b556a2800d41a3e2cfef080ac2e151178c33

Makefile

@@ -511,7 +511,7 @@ else
 	@echo "<!DOCTYPE docs SYSTEM \"appdocsxml.dtd\">" >> $@
 	@echo "<?xml-stylesheet type=\"text/xsl\" href=\"appdocsxml.xslt\"?>" >> $@
 	@echo "<docs xmlns:xi=\"http://www.w3.org/2001/XInclude\">" >> $@
-	@for x in $(MOD_SUBDIRS); do \
+	@for x in $(filter-out third-party,$(MOD_SUBDIRS)); do \
 		printf "$$x " ; \
 		for i in `find $$x -name '*.c'`; do \
 			$(PYTHON) build_tools/get_documentation.py < $$i >> $@ ; \