restrict allowable paths for bundle dir input

[SaravShah]

Hopefully i spelled all of the mounts here correctly

closes #387

[coveralls]

Pull Request Test Coverage Report for Build 1302

• 12 of 12 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.3%) to 88.103%

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Totals
Change from base Build 1294:	0.3%
Covered Lines:	511
Relevant Lines:	580

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💛 - Coveralls

[atz]

List looks correct.

[jmartin-sul]

not sure whether we want to treat the dir listing as sensitive info. guessing probably not.

still, i'd be more in favor of putting the development and test listing as a single entry in the base settings.yml (since it's the same for both envs), and then putting the production settings in production.yml in shared configs. the separate yml with env specific sections loaded by an initializer seems a little non-standard/circuitous, and i'm not sure i see the benefit.

retracting the above comment, as @atz and @SaravShah pointed out that there's some template rendering in order to get the paths set up for dev and test (i didn't notice that the config file is actually a .yml.erb, not a plain .yml).

[jmartin-sul]

what about using 'spec/test_data/../../' instead? that way, the test path's first few segments would be valid were it not for the .. (and thus more plausible as an attack if we were being more naive about our path normalization and comparison).