Validation for invalid next_run value on update

Makes sure that if next_run is manually supplied on schedule update (rather than
separate minute, hour, etc. values) that it is proper ISO 8601. Add validation
for creating a job with next_run value.

Fixes rm889
Strict enforce job_poll_interval

If job is completed quickly, make the worker wait the remainder of the
job_poll_interval config value.

Fixes rm1278
Increase image status polling time

Changed the config value so API doesn't get hit as hard.

Fixes rm1270
Merge pull request #246 from jaredculp/rm1270

Increase image status polling time
Merge pull request #240 from jaredculp/update_next_run_validation

Validation for invalid next_run value on update
Merge pull request #245 from jaredculp/enforce_job_poll_int

Strict enforce job_poll_interval
+2 lgtm
Refactor worker.py

Make code cleaner and move away from mox tests and towards Mock tests
Add polling exception with logging

Raise exception and capture error message. Bubble up to base worker class.
Merge pull request #249 from jaredculp/refactor_worker

Refactor worker