Feature: Change 'show --only-secure' to '--only-sensitive'

To be consistent with other chamber commands

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Change-Id:     Ia67fc129034dbfeff53bb518a08f99c1d2ef03ac
Signed-off-by: Jeff Felchner <accounts+git@thekompanee.com>
Refactor: Begin extracting decryption key logic into its own class

There is a lot of key logic floating around the codebase.  I'd like to
consolidate it into one or two classes.  This is the first pass at that.

Additionally, the `DecryptionKey.resolve` will always return the _contents_ of
the key and not the filename.  It does this for consistency and to allow us to
change the behavior so that we can pull the key from environment variables as
well as files on disk.

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Signed-off-by: Jeff Felchner <accounts+git@thekompanee.com>
Feature: Allow the chamber decryption key to be pulled from CHAMBER_KEY

This is extremely useful for systems like Heroku, Travis, etc.  You can set
_one_ environment variable and Chamber will pull all other settings directly
from your YAML files.

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Change-Id:     I9117d9d761b2846d87cd95738762d44be9b67fc1
Signed-off-by: Jeff Felchner <accounts+git@thekompanee.com>
Style: Fix numeric literals
Style: Fix Rubocop errors

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Change-Id:     I11ada7ddedeb2910da5591a2ed003636525181f1
Signed-off-by: Jeff Felchner <accounts+git@thekompanee.com>