cobblerd cannot start on CentOS 7 (for release26)

CentOS has a release of 7.0.1406, which is not a "float".

Use only the first two digits when assuming it is a float.

    > rpm -qf /etc/redhat-release
    centos-release-7-0.1406.el7.centos.2.3.x86_64
    > cat /etc/redhat-release
    CentOS Linux release 7.0.1406 (Core)

This is the same as #1021, rebased for the release26 branch.