[libjuju-3.1] Add provision to configure credentials via .zaza.yaml

[dashmage]

Adds feature similar to #574 to support selecting user provided credential from zaza.yaml.

Similar PR to master branch: #641

Resolves #639

[Pjack]

LGTM

[dashmage]

@ajkavanagh - could you take a look at this one as well? Thanks!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (00af63d) 89.14% compared to head (25d5b9e) 89.14%.

Additional details and impacted files

@@             Coverage Diff              @@
##           libjuju-3.1     #642   +/-   ##
============================================
  Coverage        89.14%   89.14%           
============================================
  Files               44       44           
  Lines             4689     4691    +2     
============================================
+ Hits              4180     4182    +2     
  Misses             509      509           

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[ajkavanagh]

The failures are unrelated to this change, although they do need to be addressed at some (near) stage. However, juju 3.1 support is being merged into the main branch shortly and this branch will then be deprecated. So merging this now is low risk.