Add snowflake to slack operator

[simond]

Addition of a new operator (snowflake_to_slack), the operator allows users to execute an SQL query in Snowflake and render the results into Slack. This operator renders the results of the SQL query as a Pandas Dataframe into the slack_message JINJA template variable.

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[boring-cyborg]

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Here are some useful points:

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[ad-m]

@potiuk , could you confirm that TravisCI fail is not related to that PR?

[mik-laj]

@ad-m We have temporary problems with Kubernetes tests running on Travis Ci. This change does not apply to Kubernetes, so it is safe. @dimberman is working on improvements for Kubernetes tests.

[simond]

Thanks for the review @turbaszek , just updated the PR with the changes you've suggested.

[simond]

@feluelle Thanks for the review. I've made your suggested changes and ready for another look when you get a chance!

[feluelle]

LGTM, but you only want to init the hooks once not multiple times accidently :)

[JeffryMAC]

Can the result be sent as attachment file to the message?

[boring-cyborg]

Awesome work, congrats on your first merged pull request!

[feluelle]

@JeffryMAC this can be added as feature request :)

[JeffryMAC]

@feluelle @simond submitted followup #9145