Add a graphical distinction for external networks at Topology page

Use font-awesome 'globe' glyph to indicate that network is
external. Also show a tooltip 'External Network' on hovering the
glyph.

Change-Id: I597e10a11e956a01874732bb0e9827262ebdad5d
Closes-Bug: #1381531
Merge "Add a graphical distinction for external networks at Topology page"
Merge branch 'master' of https://github.com/openstack/horizon