Investigate consistency of --slicecredfile and framework usage of get_slice_cred

[sedwards]

--slicecredfile option reads the slice credential from a file.

However this is implemented in omni.py and the underlying frameworks (especially framework_sfa.py) seem to request the slice credential independently of the code in omni.py. Double check that this is implemented correctly so that when --slicecredfile is supplied that ALL uses of the slice credential use the contents of the file.

[ahelsing]

Hmm. When you are talking to the CH, do you still want your cached credential?

To do this

• FrameworkBase?: duplicate init_user_cred substituting slice for user
• Each Framework calls this in their init method
• Each frameworks get_slice_cred must check self.slice_cred and use that first
  • maybe first check that target URN matches?
• what about if you just did a renew?