(bugfix) stop comparing strings and integers

It looks like this conditional is never being satisfied because we are
comparing a string and an int. Rather than saying `@release.to_i`, I
think it's cleaner just to say `"7"`. This will get the correct value of
`config_opts['chroot_setup_cmd']` on el7.
(bugfix) Define %dist macro for all el versions

On el7, `%dist` was being evaluated as `el7.centos`, which is not what we
want. Because of this, we need to override the definition of `%dist` on
el7 as well. Since there isn't any reason to just do this everywhere,
(i.e. not to skip el6), this also removes the conditional assignment of
`%dist`
Merge pull request #79 from melissaanne/el7-fixes

(bugfix) El7 mock config fixes