Refactor S3 credential configuration into a support function

This is in anticipation of having to make an alternate branch for IAM
instance profile support, which would otherwise bloat up and make
deeply nested branches in configure_backup_cxt.
Move STS support functions to s3_integration_help from test_aws_sts

In addition to the STS-exercising unit tests in test_aws_sts, these
support functions are useful in blackbox and simulation of AWS
instance profiles.

To reflect their shared nature, move them to s3_integration_help along
with other shared infrastructure.
Shorten default test bucket so STS policies can work

STS cannot make policies on "member" names over 32 characters.

Given the name of the test bucket is arbitrary, shorten it.
Unfortunately, it is more cryptic for purpose of inspection, but
so it goes.
Move AWS credential names to top-level in testing

To allow sharing by multiple AWS config parameter objects that may
deem it useful to scrub environment variables.
Refactor blackbox and how configuration and argv is injected

The goal is to allow the 'config' object to manipulate the argv under
test, rather than assuming the test should know how to construct argv
verbatim.

This reflects that argv can be used to set up some amount of WAL-E's
credential in addition to environment variables.

An upshot is that now that the 'main' procedure requires injection via
named fixture, also remove the magicalness of 'autouse=True'.