Uses None instead of mutables for function param defaults

As seen on #1174809, changes use of mutable types as default
arguments and defaults them within the method. Otherwise, those
defaults can be unexpectedly persisted with the function between
invocations and erupt into mass hysteria on the streets.

There was indeed a test (TestSimpleClient.test_get_with_retries)
that was erroneously relying on this behavior. Since previous tests
had populated their own instantiations with a token, this test only
passed because the modified headers dict from previous tests was
being overridden. As expected, with the mutable defaults fix in
SimpleClient, this test begain to fail since it never specified any
token, yet it has always passed anyway. This change also now provides
the expected token.

Change-Id: If95f11d259008517dab511e88acfe9731e5a99b5
Related-Bug: #1174809
debug level logs should not be translated

According to the OpenStack translation policy available at
https://wiki.openstack.org/wiki/LoggingStandards debug messages
should not be translated. Like mentioned in several changes in
Nova by garyk this is to help prioritize log translation.

Change-Id: I59486b1110f08510d83a4aec2a1666805c59d1cd
Closes-Bug: #1318333
Merge "debug level logs should not be translated"
Let users add their own txid suffixes

The value of the X-Trans-Id-Extra header on the request (if any) will
now be appended to the transaction ID. This lets users put their own
information into transaction IDs.

For example, Glance folks upload images as large objects, so they'd
like to be able to tie together all the segment PUTs and the manifest
PUT with some operation ID in the logs. This would let them pass in
that operation ID as X-Trans-Id-Extra, and then when things went
wrong, it'd be much easier to find all the requests in Swift's logs.

Also, this aids debuggability when requests fail to receive
responses. If a user is sending in their own X-Trans-Id-Extra strings,
then that gives operators something to search for in the logs. The
normal txid won't work since that's in the response, but the client
didn't receive one.

Swift will only use the first 32 characters of X-Trans-Id-Extra so
that its log lines stay a manageable length. Also, it's URL-quoted so
that users cannot inject double quotes into X-Trans-Id-Extra and screw
up log parsers.

DocImpact

Change-Id: I3c51d0c5ac55697ac230001840da219e73a03157