Updated WhitelistWatcher to watch only if a file is given.

Whitelist may be represented either by a filepath or a by a wildcard.
In the latter case WhitelistWatcher should not constantly try reload
the whitelist.

Review: https://reviews.apache.org/r/28442
Removed some DiskInfo validations from C++ Resources in favor of doing
them in the master.

Review: https://reviews.apache.org/r/28429