testing: allow lazy init of boto client

[offbyone]

Initializing the SecretsManager as an attr factory caused the client to
be initialized too early to mock it out with moto for unit testing, as
was called out in #24.

Until python-attrs/attrs#573 is fixed, we need to manually lazy-init the
client for SecretsManager

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[hynek]

I know too little about SM to meaningfully review, but code looks ok to me.

Can I haz changelog entry? Please add a "Python 2.7 support dropped" under the "breaking" section too.