Fixing sidebar layout in IE.

This problem is only reproducible when more than one panel group is
present in a dashboard (e.g., when Object Store is enabled in the
Project dashboard).  A crude way to reproduce the problem and verify
a fix is to comment out the "permissions" line in
openstack_dashboard/dashboards/project/containers/panel.py to enable
the second panel group.

The fix required was to remove an unnecessary float:left in
horizon.less that caused problems in IE's layout engine.  The
float:left had no effect on layout in Chrome, Firefox or
Safari(Windows version) in or out.

Fixes: bug #1078735
Change-Id: Idb0f8673d68ba686e1d7440fd2396e9a29b024ef

openstack_dashboard/static/dashboard/less/horizon.less

@@ -436,7 +436,6 @@ small {
 .main_nav {
     list-style: none;
     width: 222px;
-    float: left;
     margin: 10px 0 20px 0;
 }