nixos/strongswan: use strings for secrets.

The nixos module artifically enforces type.path whereas the ipsec secret configuration files
accept pattern or relative paths.
Enforcing absolute paths already caused problems with l2tp vpn:
https://github.com/nm-l2tp/NetworkManager-l2tp/issues/108

nixos/modules/services/networking/strongswan.nix

@@ -54,7 +54,7 @@ in
     enable = mkEnableOption "strongSwan";
 
     secrets = mkOption {
-      type = types.listOf types.path;
+      type = types.listOf types.str;
       default = [];
       example = [ "/run/keys/ipsec-foo.secret" ];
       description = ''