Global configuration for functions

[lance]

Allow the user to set global configuration options, for example the image registry, with func config --global.

[nainaz]

Context

Enterprises and developers using the same configuration parameters that may be different from the default ones for creating functions currently has to change the values repeatedly. Cluster admin would like to set up the common and standard parameters for the developers

Story

As Dina the developer, I should be able set some global configuration parameters across my machine/cluster so that I can have the same configuration override applied to all my functions and not fiddle with the default ones provided every time.

As cluster admin, I should be able set some global configuration parameters across my machine/cluster so that I can have the developers user the consistent approved configuration values override applied to all functions created in the org 

Acceptance Criteria

GIVEN that I provide the global override of configuration parameter 
WHEN I write and create my functions
THEN globally set configurations should be used rather than the default ones while building and deploying my function.