Add logger to graph types

[ukclivecox]

Allows logging at any point in the graph.
This feature will need to be added to the engine and its replacement the executor.
Eventually we will at v2alpha1 types but as types need to be backwards compatible we will extend v1 for now.

[ukclivecox]

/test integration

[seldondev]

Mon Dec 23 08:50:29 UTC 2019
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-1305/1.log

impatient try
jx get build logs SeldonIO/seldon-core/PR-1305 --build=1

[seldondev]

Mon Dec 23 08:50:41 UTC 2019
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-1305/2.log

impatient try
jx get build logs SeldonIO/seldon-core/PR-1305 --build=2

[seldondev]

Mon Dec 23 08:50:53 UTC 2019
The logs for [integration] [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-1305/3.log

impatient try
jx get build logs SeldonIO/seldon-core/PR-1305 --build=3

[axsaucedo]

/approve
/lgtm

[seldondev]

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