[Fix issue #15719] Add a variable for .form-group's margin-bottom

[Fix issue #15719] Add a variable for .form-group's margin-bottom

less/variables.less

@@ -207,6 +207,9 @@
 //** Small `.form-control` height
 @input-height-small:             (floor(@font-size-small * @line-height-small) + (@padding-small-vertical * 2) + 2);
 
+//** `.form-group` margin
+@form-group-margin-bottom:       15px;
+
 @legend-color:                   @gray-dark;
 @legend-border-color:            #e5e5e5;