Center .icon-prev and .icon-next

Center .icon-prev and .icon-next the same way .glyphicon icon are.
And this is the html markup:
<a class="left carousel-control" href="#carousel-main-menu" role="button" data-slide="prev">
<span class="icon-prev" aria-hidden="true"></span>
</a>
Merge pull request #15244 from zalog/patch-2

Center .icon-prev and .icon-next in carousel