Fix DNS based service discovery for memcached endpoints

[shraykay]

What this PR does: noted errors in store-gateway being unable to access memcached and found that it was related to this: https://github.com/cortexproject/cortex/pull/3203

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}