Be able to define environment variables that cut across every Stage

[kmanning]

• Right now, there are multiple plugins that let you define environment variables, but they only exist within the Stage itself (and only exist at certain "points" within the Stage)
• There are often variables that you want to set, that are consistent and visible across every stage all the time.
• One way to do this, is to set a global Jenkins variable. But global jenkins variables are not always desirable.
• Be able to set global variables that are visible across every Stage, but not visible outside of the pipeline