ha: use mysql storage for HA management

[James Page]

Vault appears to now support use of MySQL for HA storage:

  https://www.vaultproject.io/docs/configuration/storage/mysql.html#high-availability-parameters

This would avoid the need to use etcd in our deployments, and more importantly easyrsa which does not have HA in any meaningful way right now.

[James Page (james-page)]

This may be the reason we did not use MySQL for HA:

Jul 04 05:19:28 juju-82c980-disco-proposed-testing-38 vault[17065]: 2019-07-04T05:19:28.259Z [ERROR] core: failed to acquire lock: error="Error 1105: Percona-XtraDB-Cluster prohibits use of GET_LOCK with pxc_strict_mode = ENFORCING"

[James Page (james-page)]

As we direct all writes to a single node in the PXC cluster, using 'master' for pxc_strict_mode is probably OK and allows locking as required by vault.

[Nobuto Murata (nobuto)]

I'm marking this as a duplicate of LP: #1883242 since the current plan is to leverage embedded raft storage in Vault instead of implementing etcd or mysql consolidated storage backend respectively.
https://www.vaultproject.io/docs/configuration/storage/raft