bradesco: get credentials from credstash

[grvcoelho]

Description

Use credstash to get bradesco credentials (merchantId and apiKey).

closes pagarme/superbowleto-pipeline#19

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1. I've read the project's Contributing Guidelines
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3. My changes are well covered by tests and logs
4. I've updated the project docs (if needed)
5. I fell safe about this implementation
6. I feel confortable with the code I wrote, and I'm not ashamed to show it to my friends

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[grvcoelho]

fixed by #201