[RFE] support tls-certificates relation

[Dmitrii Shcherbakov]

Having certificates provided via vault or easyrsa would be good to have (for in-cluster TLS and client access).

Vault's dependence on a storage backend and storage backend's dependency on vault as a cert provider does not make it easy. One potential way to solve it would be collocation of Percona and vault "initial coordinator" units on the same logical machine and Unix socket communication for initial storage with one active unit before clustering but this is non-trivial too.

Either way, the need for this workflow exists.