Add option to use debug random

Allow Vagrant users to easily switch to using /dev/urandom instead of /dev/random to prevent kdb5_util from hanging.

Note that the documentation for the $use_debug_random was accidentally included in a previous PR and committed.  Oops.  It reads:
```
#   $use_debug_random:
#     Set to true on debug virtual systems if /dev/random
#     does not produce any data.
```