bash_app keyword arg defaults are overwritten when a bash_app is called multiple times

[benclifford]

given

@bash_app
def f(z=2):
return "echo {z}"

then the app call sequence:

f()
f(z=3)
f(z=4)
f()

will output 2,3,4,4, when it should output 2,3,4,2

I think this comes from the use of a kwargs structure that is scoped to the definition of the f app and mutated on every invocation, in parsl/app/bash.py

This branch adds a more fleshed out test case:
https://github.com/Parsl/parsl/tree/benc-1058-bash-app-kwarg-defaults

[benclifford]

fixed by #1413