Use python zipfile instead of unzip shell command

[Tomcli]

With the DLAAS June release, LCM assumes/requires all the learner images have bash unzip package which some community images such as PyTorch and Caffe don't come with it natively. Thus, as a work around, I changed the unzip command to use python zipfile because all our community images have python installed. However, we may need a better solution in the future where some images only have R or Julia for their runtimes.

@sboagibm what do you think? Thanks in advance.

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[animeshsingh]

LGTM