Remove use of git -C option from tempest venv creation

The -C option is not available in git versions older than "1.8.5" which
are still shipped by several distributions including centos 7.

Due to this incompatibility the patch has broken third party CI for
Cisco on Ironic.

Change-Id: I09a6f83f8b2fee870e6e1c50cbfdf2da4d70dfb2
1 file changed