machine: Implement 'digital-ocean-environment-type'.

* gnu/machine/digital-ocean.scm: New file.
* gnu/local.mk (GNU_SYSTEM_MODULES): Add it.
* doc/guix.texi (Invoking guix deploy): Add documentation for
'digital-ocean-configuration'.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>