Fixes #16238 - Speed up dashboard loading time.

[ohadlevy]

I've change report host to inner query and used a join, in
my testing, this speed up dramaticilly dashboard loading
time, on my setup from 60 seconds to under 10 seconds.

[tbrisker]

Nice catch! Another small change I noticed that might also help would be to change the eager_load to joins here: https://github.com/theforeman/foreman/blob/develop/app/models/host/managed.rb#L82 as I don't think it really makes any sense to use a left join here

[ohadlevy]

@tbrisker not sure, maybe @ares knows why was it initially required?

[ohadlevy]

@tbrisker regardless, I think this one worth merging without host_status optimization since host_status has a wider effect, and might need to refactor the host_status usage from the dashboard vs from host list (for example).

[tbrisker]

@ohadlevy Very well, opened http://projects.theforeman.org/issues/16245 to take care of too-eager eager-loading.

[tbrisker]

Merged as 95fd95e, thanks @ohadlevy!

[ares]

I think @dLobatog might know - 7fdb705 there also a good description in the commit.