Fix horizon build

* remove nodejs-lessc deps, use python-lesscpy;
* forward-port and update commit:8d53cf2696 from stable/grizzly
  to get rid of build-time client packages dependencies;
* minor spec improvements.

Change-Id: I928e5f9fca93e2b701d457fc23a8537ef15ec29d
Fix python-selenium package build

This package is required for horizon tests. It was broken because
pip archives contain binaries (sic!) for x86 and x86_64. Now we
remove binary for wrong architecture in custom install script.

Change-Id: I3d6e7afa6b9fcb0759d15c435b355acefbadfdaa
Require six

Nova requires six < 1.4, while many other project require just 'six',
of no particular version. Because pip-download is very straightforward
and yum is not too clever, we'd better have correct six installed
before 'prepare' action, until we can invent better fix.

Change-Id: Id849b1a7a3f7c22ecd527c78367c907dd2aed5e9