Changed template naming in fuel_agent

Made it using get_or_select_template instead of
get_template from jinja2. It allows to try several
different template names before template loader fails.
The motivation behind is that now we can have
some_ubuntu_1204_x86_64.jinja2
some_ubuntu.jinja2
some.jinja2
templates in order to make profile scheme more flexible.

Change-Id: I504cbeb1670966690887e02201286a3dff952284
Merge "Changed template naming in fuel_agent"