Implement checks for CentOS 7 in install

http://tracker.ceph.com/issues/9041
a bare except is dangerous, if it fails we would like to know how/why/ it failed and this will eat anything and everything silently.

At the very least, if you think this would be so brittle that it would break often, log a warning on it with the exception. You can even log the exception itself. The `distro` object has a logging instance, so you could pass `distro` here and use `distro.release` and then log with `distro.conn.logger`:

    try:
        ...
    except:
        distro.conn.logger.exception('could not detect correct version for this release')

However, as I write this, we shouldn't even try to continue at this point. Slapping a '0.0' would break things further
down. It is entirely reasonable to just error here with a meaningful message and let that exception bubble up.