Set default for bootstrapExpect to replicas

[lkysow]

In almost all cases, users want to set bootstrapExpect to the number of
server replicas. This change defaults it to null in values.yaml and then
in the template if it's left as null, then we set the -bootstrap-expect
flag to the number of server replicas.

This is backwards compatible, if users have been setting this it will
continue to be set.

Also error out if bootstrapExpect is less than the number of replicas
because this is definitely a misconfiguration as the servers won't wait
until the proper number have started before electing a leader.

Before:

server:
  replicas: 1
  bootstrapExpect: 1

After:

server:
  replicas: 1

Closes #573

[thisisnotashwin]

Is this change backwards compatible? Previously, we would allow bootstrap expect to be <= replicas but this change would cause such a setting to change.

[lkysow]

Ooh, you're right! Will add to changelog. I could also remove this but I think this is an invalid configuration.

[thisisnotashwin]

Interesting. I mean, your comment about bootstrapExpect made me realize that my understanding of it was incorrect. I thought it was the min number of servers that needed to be up and running for consensus, which seemed odd but 🤷 . This makes more sense.

[lkysow]

To prevent inconsistencies and split-brain (clusters where multiple servers consider themselves leader) situations, you should either specify the same value for -bootstrap-expect or specify no value at all on all the servers.

From https://www.consul.io/docs/install/bootstrapping

[kschoche]

Excellent change.