etree: add support for xml validation

This patch adds support for XML validation with RelaxNG.  The original S3
schema is available at http://doc.s3.amazonaws.com/2006-03-01/AmazonS3.xsd, but
I write a schema for swift3 from scratch.  It is because:

 - The original schema does not support all of the latest S3 API.  Even if we
   use it, we have to update and maintain it in either way.

 - The original schema is written with XML Schema, but the language is not
   enough to describe some of the latest S3 API.  For example, the Multi-Object
   Delete operation sends an XML document, which can interleave a Quiet element
   with Object elements.  XML Schema cannot define such kinds of XML documents.

This patch includes both RelaxNG (rng) and RelaxNG compact syntax files (rnc)
for validation.  What I wrote are only rnc files, and rng files are
automatically generated from rnc with trang, the most well-known schema
converter.  If possible, I'd like to use only rnc schemas since rng files are
complicated and not so human-readable.  However, lxml doesn't support RelaxNG
compact syntax currently, unfortunately.  After lxml supports compact syntax,
let's remove all of the rng files.

This patch also fixes some XML errors detected by RelaxNG validator, and some
helper methods to convert strings between camel-case name and snake-case name.

Change-Id: I4513345a6b981efc5d5f5e8cf528aba84ac1bdad
Merge "etree: add support for xml validation"