Allow predictor replica changes to not cause rolling update

[ukclivecox]

Fixes #2008

• Ensure predictor.replicas is zeroed when taking bas64 representation for executor. This ensure pod does not change just for this reason.
• Adds test in rolling_updates notebook

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[seldondev]

Fri Jun 26 10:48:09 UTC 2020
The logs for [pr-build] [1] will show after the pipeline context has finished.
https://github.com/SeldonIO/seldon-core/blob/gh-pages/jenkins-x/logs/SeldonIO/seldon-core/PR-2016/1.log

impatient try
jx get build logs SeldonIO/seldon-core/PR-2016 --build=1

[seldondev]

Fri Jun 26 10:48:06 UTC 2020
The logs for [lint] [2] will show after the pipeline context has finished.
https://github.com/SeldonIO/seldon-core/blob/gh-pages/jenkins-x/logs/SeldonIO/seldon-core/PR-2016/2.log

impatient try
jx get build logs SeldonIO/seldon-core/PR-2016 --build=2

[axsaucedo]

/test notebooks
/approve

[seldondev]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: axsaucedo

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[seldondev]

Mon Jun 29 13:13:36 UTC 2020
The logs for [notebooks] [3] will show after the pipeline context has finished.
https://github.com/SeldonIO/seldon-core/blob/gh-pages/jenkins-x/logs/SeldonIO/seldon-core/PR-2016/3.log

impatient try
jx get build logs SeldonIO/seldon-core/PR-2016 --build=3

[seldondev]

@cliveseldon: The following test failed, say /retest to rerun them all:

Test name	Commit	Details	Rerun command
notebooks	7b491d7	link	/test notebooks

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