.form-control:focus customization issues

Created by: NicoLB

I spotted a couple of issues regarding .form-control:focus state.

1. Missing !default on a couple of variables making it impossible to customize them in _custom.scss

$input-bg-focus: $input-bg;
$input-color-focus: $input-color;

2. Inconsistency with the variables generating shadows _variables.scss

$input-box-shadow: 0 1px 1px rgba(0,0,0,.075) !default;
$input-box-shadow-focus: rgba(102,175,233,.6) !default;

mixins/_forms.scss

@mixin form-control-focus() {
  ...
  $shadow: inset 0 1px 1px rgba(0,0,0,.075), 0 0 8px $input-box-shadow-focus;
  ...
}