New `module/softlayer'

New site module that interfaces with `extern/softlayer'.
Updated `libexec/softlayer': flag for cache added

Added a programatic flag (variable) to enable/disable caching, and
set that to false as softlayer API calls are not too costly.
New `module/mongo': mongodb site module
Amended `bin/ssh': persist on down hosts

If a host is not responding on port 22, then keep trying it until it
does - and only then make an ssh connection attempt.

This is now used in `::remote:tmux()' instead of the ssh binary
itself.
Updated `module/gpg': minor update
Added `module/util' unit tests: for nima/site#4