docker created from fairing uses python2 even base docker image has python 3 as default

[Akashdesarda]

/kind bug

What steps did you take and what happened: I am using Trainjob API to deploy and run it remotely on GKE cluster. Following is the flow:

1. After running Trainjob API it is first creating docker (working correctly)
2. Then copying all the necessary dependencies and packaging it with docker (I am still not sure about this step) 3.Then pushing docker to specified registry (working correctly)
3. Then using the same docker to run it on GKE cluster (from here the problem starts) In step no 4 folling is the problem

Traceback (most recent call last):
  File "/app/function_shim.py", line 4, in <module>
    from enum import Enum
ImportError: No module named enum
```   
I looked up the problem & following is conclusion:
1. enum package generally gives problem in python2
2. So I made sure my base docker image will use python3 as default. So should the docker generated step 2, but I think this is not the case. 

**What did you expect to happen:**
Function inside the docker should work just like the function working locally. 

**Anything else you would like to add:**
Docker generated in step 2 i.e. by Faring should use python3 by default 
I think so make change in Docker command python to python3.  (below log may help)
`Docker command: ['python', '/app/function_shim.py', '--serialized_fn_file', '/app/pickled_fn.p']`



**Environment:**

- Fairing version: 0.5.3
- Kubeflow version: 0.5

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[jinchihe]

@Akashdesarda Pls see my suggestion in the https://github.com/kubeflow/fairing/pull/333 thanks.

[shikha130vv]

Ok so it is because the environment in which fairing code is running, python is pointing to python2. Yes, @jinchihe suggestion to make python point to python3 will solve this issue.

[jtfogarty]

/priority p2 /area engprod