use Circle CI as dedicated CI for acs-engine, make CI results public

[seanknox]

What this PR does / why we need it:

This PR introduces Circle CI as a public replacement for our private Jenkins server. This will allow all acs-engine contributors to see CI build information.

Some immediate advantages to this setup:

• CI results can be viewed publicly, not just by Microsoft employees.
• CI configuration is checked in to source control, viewable/editable by any member of the team.
• Sensitive data like service principal credentials are encrypted and not viewable once encrypted.
• Unit/style/code coverage checks have been split out from e2e tests and run as independent stages in the pipeline. This allows us to run them in parallel. In addition, if e2e fails, we can re-run just that stage instead of starting over.
• We've configured e2e test runs in Circle CI to still require approval from an acs-engine team member. Unit tests run automatically, but we can turn that off too if we like.
• Circle CI's excellent UX makes clear what and how many builds are in the queue, what failed, and other key information for contributors.

Which issue this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged): fixes #1134

Special notes for your reviewer:

Release note:

use Circle CI as dedicated CI for acs-engine

cc @anhowe @dmitsh @dtzar

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This change is 

[seanknox]

Will rebase after #1146 goes in.

[seanknox]

Example with Circle CI webhooks is at https://github.com/seanknox/acs-engine/pull/3

[seanknox]

Circle is a bit confused by the webhooks in my fork. Closing and re-opening with a branch on this repo.

[seanknox]

Moved over here: #1148