This patch adds "--output-dir" and "--remove-prefix" options
to the "download" command and unit tests for it.

Example:

    $ swift list example --prefix swift2.2
    swift2.2/bin/swift-object-auditor
    swift2.2/bin/swift-object-expirer
    swift2.2/bin/swift-object-info
    swift2.2/bin/swift-object-replicator
    swift2.2/bin/swift-object-server
    swift2.2/bin/swift-object-updater

When given "--output-dir <directory>", client downloads objects to <directory>.

    $ swift download example --prefix swift2.2 \
                    --output-dir new/swift/dir

    The folder structure:
    .
    └── new
        └── swift
            └── dir
                └── swift2.2
                    └── bin
                        ├── swift-object-auditor
                        ├── swift-object-expirer
                        ├── swift-object-info
                        ├── swift-object-replicator
                        ├── swift-object-server
                        └── swift-object-updater

When given "--remove-prefix", client downloads objects without <prefix>.

    $ swift download example --prefix swift2.2 \
                    --remove-prefix \
                    --output-dir swift

    The folder structure:
    .
    └── swift
        └── bin
            ├── swift-object-auditor
            ├── swift-object-expirer
            ├── swift-object-info
            ├── swift-object-replicator
            ├── swift-object-server
            └── swift-object-updater

Co-Authored-By: Clay Gerrard <clay.gerrard@gmail.com>
Change-Id: I7463fe2941cc94f9a50a4756a97c2ccdf946294d
Implements: blueprint swiftclient-download-pseudo-folder-to-specific-target

There was always an implicit assumption that they were UTF-8 before, and
by converting them to unicode we close another hole allowing raw bytes
to appear in user-facing messages.

Closes-Bug: #1431866
Change-Id: If2e41d9a592c3ad02818e9c6f0959fd4b73cd0e0

Previously we were trying to test writing bytes in Python 3 using only
native (unicode) string objects. That doesn't test what we thought we
were testing.

Change-Id: I10a0a38143d7f7d850ab9a7005ad87f5d314c375