charmhelpers/core/templating.py only imports python-jinja2, useless when using python3

[Francis Ginther]

This code in charmhelpers/core/templating.py will install 'python-jinja2' if it's not otherwise installed:

def render(source, target, context, owner='root', group='root',
           perms=0o444, templates_dir=None, encoding='UTF-8', template_loader=None):
    """..."""
    try:
        from jinja2 import FileSystemLoader, Environment, exceptions
    except ImportError:
        try:
            from charmhelpers.fetch import apt_install
        except ImportError:
            hookenv.log('Could not import jinja2, and could not import '
                        'charmhelpers.fetch to install it',
                        level=hookenv.ERROR)
            raise
        apt_install('python-jinja2', fatal=True)
        from jinja2 import FileSystemLoader, Environment, exceptions

However, when run as part of a python3 hook, it needs to import 'python3-jinja2' instead.