Fix install_venv-get_distro failure on Fedora

There was no check_exit_code=False used so install_venv died
on Fedora/RHEL/etc.
Also calling external grep command is not neccessary.

Change-Id: I280aed3019b8cb7fce67033158deb34937946fab
Merge "Fix install_venv-get_distro failure on Fedora"