Export environment variable for "startFrom"

[jiantian]

What happened:

A screwdriver job be started by commit, or by buildPeriodically . However, there is no environment variable to define where the job starts from. It will be useful to export such variable since our SD job might have different logics depending on how the job gets started.

What you expected to happen:

Simply add an environment variable like startFrom will suffice.

How to reproduce it:

[AkMo3]

@jithine @tkyi I am willing to solve this issue. I believe editing add environment variable SD_START_FROM variable in launch.go and launch_test.go . Please let me know your views on this solution.

[jithine]

startFrom is already a part of the build event.