Fix item assignment issue with ConfigSecretVolume

[Molaire]

This PR fixes the schema assumption that configsecretVolume's "items" field is an array. I did not find the source of the array to tuple transformation, but this is the only place in the code that assume it's a writable array, so this also resolve the problem.

This test job was used with a locally modified version of tron in infrastage.

BEFORE change: https://fluffy.yelpcorp.com/i/7h1Tdwvblkf4KKKxz74V4T83XSbDxfsL.html
run successfully on infrastage: https://fluffy.yelpcorp.com/i/RWBcgXWqVfcsrFVXf9rtwfN6B9N7RpT4.html
retries successfully on infrastage: https://fluffy.yelpcorp.com/i/0d9hRM3hQ3KWv0nSDcHtTKGDLK7phXpt.html

The (currently) failing itest is also present for working commits in master. I don't know what this is about.

[danielpops]

lgtm

>>> l = list(('a', 'b'))
>>> l[1] = 'new'
>>> l
['a', 'new']

[nemacysts]

imo, we probably want a more descriptive comment for future yelpers - but lgtm otherwise :)