allow state_top_from_file to specify which file to use as top.sls

[anuriq]

First, mentioned here
#92 (comment)

After reading docs for state_top_from_file, it seems like i could provide a path to a file and that file will be used as top.sls, but in reality (after checking the code) this is just a trigger true/false. Maybe there should be a possibility to provide path to a file, which should be used as top.sls?

I could work on PR, if you agree.

[gtmanfred]

I am going to fix the docs, because I am about to do a release today, but I would like to see this changed to allow it to also be a file to use.

[gtmanfred]

I updated the doc to be correct.

#190