Manual pipelines should support predefined variables

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We should be able to define variables on the section "Run a pipeline manualy". This could be handy when you want to debug with it, just need to run a pipeline with the var CI_DEBUG_TRACE.

Related to https://gitlab.com/gitlab-org/gitlab-ce/issues/2989#note_27062708 https://gitlab.com/gitlab-org/gitlab-ce/issues/30610

/cc @zj @markpundsack