Feature/msp 9842/arel helper

# Verification Steps

- [x] `bundle install`

## `rake spec`
- [x] `rake spec`
- [x] VERIFY no failures
- [x] `cd spec/dummy`
- [x] run `bundle exec rails console`
- [x] `puts Mdm::Host.joins(Mdm::Host.join_association(:services)).to_sql` should output
SELECT "hosts".* FROM "hosts" INNER JOIN "services" ON "services"."host_id" = "hosts"."id"
- [x] lets test something weird. run `puts Mdm::Host.joins(Mdm::Host.join_association(:services, Arel::Nodes::OuterJoin)).select(Mdm::Service[Arel.star]).to_sql`
it should output
`SELECT "services".* FROM "hosts" LEFT OUTER JOIN "services" ON "services"."host_id" = "hosts"."id"`