Add HA lock storage support

[Liam Young]

The vault charm currently has no HA support for the vault service itself. The vault documentation makes a distinction between two types of information “Vault's information” and the “HA lock”. Vaults information can be stored in any of the backends listed here https://www.vaultproject.io/docs/configuration/storage/index.html but the HA lock can only be stored in Consul, DynamoDB, Etcd or zookeeper. Vault’s information can be stored alongside the HA lock in one of those four datastores too if required.

To support HA the vault charm will need to grow a new interface for storing the HA lock.

I think etcd support would be a good place to start