Temporary PR to integrate PDL's mass of bug fixes and enhancements

[devanshk]

The follow two bugs, combined, prevent pending jobs from being executed:

• When number of jobs is larger than number of vms in free pool, jobManager dies.

• When jobManager restarts, free pool is not emptied whilst total pool is, causing inconsistency.
  xyzisinus@4dcbbb4
  xyzisinus@e2afe8a

• Add ability to specify image name for ec2 using "Name" tag on AMI (used to allow only one image specified as DEFAULT_AMI):
  xyzisinus@97c22e3
  xyzisinus@e66551a

• When job id reaches the max and wraps around, the jobs with larger ids starve.
  xyzisinus@9565275

• Improve the worker's run() function to report errors on the copy-in/exec/copy-out path more precisely.
  xyzisinus@caac9b4
  xyzisinus@c47d889

• In the original code, Tango allocates all vm instances allowed by POOL_SIZE at once. It shouldn't be an issue because once a vm is made ready a pending job should start using it. However, due to well-known Python thread scheduling problems, the pending jobs will not run until all vms are allocated. As we observed, vm allocations are almost sequential although each allocation runs in a separate thread, again due to Python's threading. That results in a long delay for the first job to start running. To get around the problem, POOL_ALLOC_INCREMENT is added to incrementally allocate vms and allow jobs to start running sooner.
  xyzisinus@93e60ad

• With POOL_SIZE_LOW_WATER_MARK, add the ability to shrink pool size when there are extra vms in free pool. When low water mark is set to zero, no vms are kept in free pool and a fresh vm is allocated for every job and destroyed afterward. It is used to maintain desired number of ec2 machines as standbys in the pool while terminating extra vms to save money.
  xyzisinus@d896b36
  xyzisinus@7805577

• Improve autodriver with accurate error reporting and optional time stamp insertion into job output.
  Tango/autodriver/autodriver.c

• When Tango restarts, vms in free pool are preserved (used to be all destroyed).
  xyzisinus@e2afe8a

• Add run_jobs script to submit existing student handins in large numbers:
  Tango/tools/run_jobs.py

• Improve general logging by adding pid in logs and messages at critical execution points.

Part 2. New configuration variables (all optional)

• Passed to autodriver to enhance readability of the output file. Currently only integrated in ec2 vmms.
  AUTODRIVER_LOGGING_TIME_ZONE
  AUTODRIVER_TIMESTAMP_INTERVAL

• Control of the preallocator pool as explained in Part 1.
  POOL_SIZE_LOW_WATER_MARK
  POOL_ALLOC_INCREMENT

• Instead of destroying it, set the vm aside for further investigation after autodriver returns OS ERROR. Currently only integrated in ec2 vmms.
  KEEP_VM_AFTER_FAILURE

[victorhuangwq]

@fanpu, this PR seems to contain a lot of the important fixes that Tango really needs.

I think the ideal way to go about adding this PR into Tango is to break this PR down into smaller more easier to review PRs.

I'm currently thinking that we have 1 PR based on each commit (that is, each of the commits based in the description above), so that we can actually logically get people to approve and merge them in.