Function calls for secured and slack environment variables

[hsiliev]

We now use function calls for secured and slack in most components, instead of pure constants.

AFAICT we use functions to make setting up test environments easier by simply changing the SECURED and SLACK variables.

This has a small negative performance overhead so we might consider using constants instead. This will indeed complicate the tests a bit but will also extract the environment setup in before blocks. The drawback will be that we'll need to delete the require cache as well.

[cf-gitbot]

We have created an issue in Pivotal Tracker to manage this:

https://www.pivotaltracker.com/story/show/133526285

The labels on this github issue will be updated when the story is started.