nixos/cfssl: init

- based on module originally written by @srhb
- complies with available options in cfssl v1.3.2
- uid and gid 299 reserved in ids.nix
- added simple nixos test case

nixos/modules/misc/ids.nix

@@ -323,6 +323,7 @@
       mapred = 296;
       hadoop = 297;
       hydron = 298;
+      cfssl = 299;
 
       # When adding a uid, make sure it doesn't match an existing gid. And don't use uids above 399!
 
@@ -606,6 +607,7 @@
       mapred = 296;
       hadoop = 297;
       hydron = 298;
+      cfssl = 299;
 
       # When adding a gid, make sure it doesn't match an existing
       # uid. Users and groups with the same name should have equal