Related-bug: #1413919 Related-bug: #1413923
Related-bug: #1413919
Related-bug: #1413923

Issue: In "Configure > Security Groups" page, on clicking Address column,
dropdown popup is not aligned properly w.r.t drop-down and is positioned
a bit above the drop-down [when drop-down popup has no sufficient space
on bottom]

Instead of modifying select2 drop-down popup css from javascript, added
dropdownCss class for select2 and moved the css settings to
contrail.custom.css

Modified select2 to set the bottom instead of setting top in case of drop-down popup shown
above the drop-down. Setting top won't work in all cases especially if
drop-down height is not constant like having collapse/expand containers
within it.

Change-Id: I3e29331423c26179a75ab526c3a0ac480f1e5735

sbavanasi/contrail-web-controller,sbavanasi/contrail-web-controller,vishnuvv/contrail-web-controller,biswajit-mandal/contrail-web-controller,vishnuvv/contrail-web-controller,sbavanasi/contrail-web-controller,skizhak/contrail-web-controller,varunarya10/contrail-web-controller,biswajit-mandal/contrail-web-controller,JioCloud/contrail-web-controller,vishnuvv/contrail-web-controller,biswajit-mandal/contrail-web-controller