Fix TestStoreGateway_InitialSyncWithWaitRingStability flakyness

[pracucci]

What this PR does:
The TestStoreGateway_InitialSyncWithWaitRingStability is flaky and fails with this error:

=== RUN   TestStoreGateway_InitialSyncWithWaitRingStability/shuffle_sharding_strategy,_20_gateways,_RF_=_3,_SS_=_3_(bucket_index_enabled_=_false)
    gateway_test.go:300: random generator seed: 1624370314782121000
    gateway_test.go:347:
        	Error Trace:	gateway_test.go:347
        	Error:      	Received unexpected error:
        	            	invalid service state: Failed, expected: Running, failure: unable to start store-gateway dependencies: not healthy, 0 terminated, 1 failed: [register instance in the ring: failed to CAS store-gateway]
        	Test:       	TestStoreGateway_InitialSyncWithWaitRingStability/shuffle_sharding_strategy,_20_gateways,_RF_=_3,_SS_=_3_(bucket_index_enabled_=_false)

We've experienced a similar issue CASing the in-memory consul in the test TestShuffleShardWithCaching and, at that time, we solved it increasing retries and adding a delay between retries. Getting back to TestStoreGateway_InitialSyncWithWaitRingStability, I'm able to reproduce the issue locally in master but I can't reproduce it after this PR's change 🤞

Which issue(s) this PR fixes:
Fixes #4290

Checklist

• Tests updated
• Documentation added
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]