virt: Replace deprecated template with eval in cart-config.

Replace templates.subtitution by eval in Cartesian config.
It allows use basic python expression in configuration code.
Speed is almost same as with tempaltes.subtitution.

Example:
 cfg:
   mem = 1000
   real_mem = ${float(mem) * 0.7}M

 output:
    mem = 1000
    real_mem = 700M

Signed-off-by: Jiří Župka <jzupka@redhat.com>