Support virtualenv installation in baremetal role

Installing python packages directly to the system site-packages can
cause various problems, in particular when pip overwrites a system
package. Python virtualenvs are one solution to this issue, as they
allow python packages to be installed in an isolated environment.

This change adds support to the baremetal role for installing python
dependencies in a virtualenv. Typically we will need to enable use of
system site-packages from within this virtualenv, to support the use of
modules such as yum, apt, and selinux, which are not available on PyPI.

The path to the virtualenv is configured via the 'virtualenv' variable,
and access to site-packages is controlled via
'virtualenv_site_packages'.

When executing other kolla-ansible commands, the variable
'ansible_python_interpreter' should be set to the python interpreter
installed in 'virtualenv'. Note that this variable cannot be templated.

Change-Id: I0741923065246f9c5b168059fcd66504f2753c41
Related-Bug: #1731026