raft: upgrade to v1.7.0 which introduces pre-vote extension

[jrasell]

In the near future, Raft will release v1.7.0 which introduces the pre-vote extension: hashicorp/raft#530

The feature particularly improves cluster stability when a Nomad server is partition from the cluster, but rejoins at a later date. The PR has a conclusive writeup.

In order to update Nomad we will want to decide if the new preVoteDisabled configuration flag is exposed to operators and perform testing. The testing should include:

• upgrade testing with mixed Raft versions
• leadership transitions
• server partitioning and rejoining
• pre-vote interactions with Autopilot
• load testing