tests.cfg: Set up the lowest_mem for unattended_install

As we try to shrink memory size when host don't have enough huge pages.
unattended_install may fail because guest don't have enough RAM in Linux guests
when we start guest with huge pages. This is caused by anaconda memory size
check. For newest RHEL and fedora the lowest memory size is 512M. So set up the
it to 512M in cfg file.

Signed-off-by: Yiqiao Pu <ypu@redhat.com>
Merge pull request #823 from ypu/low

 tests.cfg: Set up the lowest_mem for unattended_install