Fix image Jenkinx X build failures

[axsaucedo]

Fixes #2443

The jenkins X uses the VERSION branch as the base branch, so when it was changed to used nighly-<version> it broke the image build, resulting in the wrong branch being built. Now this builds on the correct branch back to v<version>

[seldondev]

Thu Sep 17 12:58:01 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-2449/2.log

impatient try
jx get build logs SeldonIO/seldon-core/PR-2449 --build=2

[seldondev]

Thu Sep 17 12:58:01 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-2449/1.log

impatient try
jx get build logs SeldonIO/seldon-core/PR-2449 --build=1

[RafalSkolasinski]

Should also fix #2444

[RafalSkolasinski]

/lgtm

let's merge it and see if all problems go away!

[RafalSkolasinski]

/approve

[seldondev]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: RafalSkolasinski

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[seldondev]

Thu Sep 17 13:21:03 UTC 2020
The logs for [pr-build] [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-2449/3.log

impatient try
jx get build logs SeldonIO/seldon-core/PR-2449 --build=3

[seldondev]

Thu Sep 17 13:21:08 UTC 2020
The logs for [lint] [4] will show after the pipeline context has finished.
https://github.com/SeldonIO/seldon-core/blob/gh-pages/jenkins-x/logs/SeldonIO/seldon-core/PR-2449/4.log

impatient try
jx get build logs SeldonIO/seldon-core/PR-2449 --build=4