Revise to be inline with Cluster Manager interface

[omus]

Fixes https://github.com/beacon-biosignals/K8sClusterManagers.jl/issues/13

Overview of the changes:

• Renamed K8sNativeManager to K8sClusterManager.
• Since the addprocs function typically handles specifying defaults for exename and exeflags I needed to move the worker pod spec generation into the launch method. Doing this required changes to what was stored within the K8sClusterManager struct.
• If a user attempts to launch a k8s cluster without specifying an image while running the code outside of a pod an exception will be thrown. This check is now done in the K8sClusterManager constructor to avoid having the exception raised inside of a task (how launch is called).
• Added a K8sClusterManager constructor that takes number of worker to launch
• Stored the pod name in the worker config userdata inside a NamedTuple. Should allow for additions without breaking the code.
• Only create a single KuberContext instance while testing (reduces noise)
• Avoid defining kill and manage methods for our cluster manager as we're not adding value. The default kill will remotely call exit on the workers which allows the worker pods to complete instead of always error.

[omus]

Really glad I added those Julia 1.3 tests in. I used a syntax that's only available on Julia 1.5+

[omus]

I'll work through the CI failures tomorrow as I'm out of time for today. I'll note that I'm not seeing them locally.

[omus]

I should have fixed the CI issue now. I seem to be unable to re-use the KuberContext in environments where I can't successfully connect to the Kubernetes API: https://github.com/JuliaComputing/Kuber.jl/issues/27

[codecov-commenter]

Codecov Report

Merging #31 (87ef696) into main (8ede3ab) will decrease coverage by 13.02%. The diff coverage is 12.50%.

@@             Coverage Diff             @@
##             main      #31       +/-   ##
===========================================
- Coverage   40.70%   27.67%   -13.03%     
===========================================
  Files           4        4               
  Lines         113      112        -1     
===========================================
- Hits           46       31       -15     
- Misses         67       81       +14     

Impacted Files	Coverage Δ
src/K8sClusterManagers.jl	81.81% <ø> (-1.52%)	:arrow_down:
src/native_driver.jl	4.61% <11.53%> (-3.45%)	:arrow_down:
src/pod.jl	15.00% <14.28%> (-50.22%)	:arrow_down:

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[omus]

I've updated codecov to use main as the origin/HEAD.

[omus]

I'm going to tackle test coverage in another PR