Fixes #12233 - clean the interface cache only when becoming Managed::Host

[iNecas]

Without the patch, discovery reboot fails on

```NoMethodError: undefined method`drop_execution_interface_cache' for

[ares]

maybe use became.respond_to?, I'm not sure what host types we'll ever see :-) Otherwise 👍

[stbenjam]

Isn't this more a problem of us extending the descendants of Host::Base? I still don't think we need to do that, the UI doesn't allow executing jobs on discovery hosts. That might be worthwhile eventually in the Discovery case, but like @ares said, we don't know what host types we'll ever see, yet we assume now we should extend them all with our remote execution methods.

[iNecas]

No, actually, this is hapending with your PR https://github.com/theforeman/foreman_remote_execution/pull/55/files#diff-24c9f52e5c8d0dc8ba4b5d2b7ec5de44R107, I noticed that, when I was testing the #55, So in master, it actually might work even now

[iNecas]

@stbenjam I'm ok with closing this PR if you're going to address that in your PR (until now, I haven't realized that issue might be related to the #55)

[stbenjam]

Yea @ares ran into a similar problem on another PR and we had a discussion about it, the outcome was to keep the descendants but remove Host::Base itself, but this is a good example of why not to do the descendants as well.

@ares is there a reason to keep the descendants you think? If so we can take this and I can change #55, I don't really have a strong opinion about it

[stbenjam]

Per discussion on hangouts, APJ from me

[stbenjam]

Filed http://projects.theforeman.org/issues/12240