Change how and when we fetch and parse namespace info

[Waterdrips]

Description

The initial namespace select implementation had some challenges, especially around non-default installs (no openfaas-fn namespace)
This has removed lots of those issues and added some more user friendly bits too (defaulting to first returned namespace from backend, passing that namespace to the function-create as default)

Signed-off-by: Alistair Hey [email protected]

Motivation and Context

• I have raised an issue to propose this change (required)
• My issue has received approval from the maintainers or lead with the design/approved label
  closes Namespace selector does not update the function list  #1408
  closes Namespace selector does not update the function list  #1408

How Has This Been Tested?

kubernetes:
Install with 4 active namespaces, switching works, adding a fn works (for each namespace)
Can add the same fn to each namespace
Can call each fn, see it calls correct one in logs
Can delete functions

Swarm:
No NS selector, can create, call and delete fns.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I've read the CONTRIBUTION guide
• I have signed-off my commits with git commit -s
• I have added tests to cover my changes.
• All new and existing tests passed.

[alexellis]

Merging based upon review from @LucasRoesler