restrict allowable paths for bundle input dir

[jmartin-sul]

we should restrict to the mount paths on the pre-assembly box.

@blalbrit can provide a whitelist of valid input paths.

[jmartin-sul]

this is something of a security issue. we also want to make sure they don't do sneaky things like going from a whitelisted root path but then backing up using relative dirs via .. etc.

[jmartin-sul]

for tests, the rspec spinup can add the fixture paths to the whitelist. then, don't have to do anything env dependent in regular app code.

[blalbrit]

The active mounts are listed at https://consul.stanford.edu/display/CollAcc/Content+Mount+Paths (mount path column for the path that is accessed from the prod preassembly box)