Install packages required to build clients

These were dropped when the package support was added, they are not
required for package builds but for now there is no way to select an
element wide install script based on install type. So we install in
all cases.

Also adding 3 new dependencies(python-virtualenv, libxml2-dev and
libxslt-dev) that were never listed but are required.

Closes-Bug: #1274846
Change-Id: I8eb04b6d1527de43b768e2b51a1c916202ea5ee9
Merge "Install packages required to build clients"