fix(boot.go,jobs/jobs.go): fix panic on startup

[arschles]

Summary Of Issues

I opened this pull request to fix the panic described in #54. After fixing the panic described in that issue, I uncovered the following other bugs, in order of the below listing. Since each bug uncovers the next in turn, I believe that the fixes all need to be included into a single PR, even if this PR is large in scope.

• The code was passing an incorrect time.Duration to the poller
  • It was effectively passing time.Duration(config.Spec.Polling)*time.Second*time.Second into time.NewTicker, which was a negative number and caused time.NewTicker to return an error
• The first recurring job is infinitely recurring (see #55, TODO below)
• The VERSIONS_API_URL and DOCTOR_API_URL environment variables (among others) are not being set properly in the deis chart
  • PR forthcoming for that issue. For now, see https://github.com/deis/charts/issues/267 (fixed by https://github.com/deis/charts/pull/268) for tracking

    Summary of Fixes

This PR include the following fixes to address the above issues:

• Creates a new rest package that has a Client interface, whose implementation carries a shared *http.Client and the base URL for the workflow-manager API server. As such, that data need not be passed everywhere the client is used
• Specifies defaults in the envconfig struct for the VERSIONS_API_URL, DOCTOR_API_URL, and API_VERSION environment variables. These defaults allow this component to function until https://github.com/deis/charts/issues/267 is fixed
• Refactors the interface implementations in the data package to be non-exported. This change forces other packages to create these implementations via "constructor" functions, which ensures that all required fields are given. This design pattern forces all other packages to create new implementations in a single place, which greatly reduces the chances that other packages initialize an implementation with nil members, and fixes #54
• Fixes the infinite recursion specified in #55 by passing a types.Cluster to (github.com/deis/workflow-manager/data).AvailableVersions's Refresh func. With this change, AvailableVersions no longer needs to call GetCluster, which was the source of the recursion
• Changes various implementations in the data package to no longer swallow errors. All code in data should now check for and return errors
• Similar to the above, code in the data package no longer logs errors. Instead, implementations just return the errors and rely on callers to handle logging, handling, etc...
• Passing a time.Duration to jobs.DoPeriodic and not multiplying that time.Duration by time.Second. Previously, the resultant time.Duration was an overflow and time.NewTimer was returning an error
• Creating tests for jobs/jobs.go

Fixes https://github.com/deis/workflow-manager/issues/55 Fixes https://github.com/deis/workflow-manager/issues/54

Still TODO

• [x] fix throttled reqs (see https://github.com/deis/workflow-manager/issues/55 for what logs look like)
• [x] fix tests

[mention-bot]

By analyzing the blame information on this pull request, we identified @jackfrancis to be a potential reviewer

[bacongobbler]

From my naive surface-deep code review (not too well versed with workflow-manager... Need to do a deep dive one day), this all looks good to me.