etcd_docker 2: Add a docker based etcdintegration package

[andrewmains12]

PR 2 for #4144

High level approach is as described in #4144 . This PR adds:

• Functions to spin up a 1 node etcd cluster using docker (in dockerexternal)
• A drop in replacement for the etcd/integration package using dockerexternal

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Stack:

• etcd_docker 7: move etcd dependency down to 3.5.4 (#4151) #4209
• etcd_docker 6: Remove etcd embed from M3DB node #4208
• etcd_docker 5: Incorporate docker based etcd approach into docker integration tests. #4207
• etcd_docker 4: Incorporate docker based etcd into Go integration tests (#4148) #4206
• etcd_docker 3: Incorporate docker based etcd integration package into… #4205
• etcd_docker 2: Add a docker based etcdintegration package #4154 ⬅

⚠️ Part of a stack created by spr. Do not merge manually using the UI - doing so may have unexpected results.