Allow mTLS for mysql secrets engine

[Valarissa]

This change enables the usage of mTLS with the mysql secrets engine.

It requires the following values to be passed to the MySQLConnectionProducer in order to enable mTLS:

	TLSCertificateKeyData []byte `json:"tls_certificate_key" mapstructure:"tls_certificate_key" structs:"-"`
	TLSCAData             []byte `json:"tls_ca"              mapstructure:"tls_ca"              structs:"-"`

These provide a combined Cert and Key PEM block and the CA PEM block of the CA that signed the server and client certs.

An automated test is provided in the code presented, it is skipped by default due to our CI not being able to create and mount files to a docker image.

Feel free to ask any questions to help review this.

Test output:

go test -run "clientTLS" -v ./plugins/database/mysql/
=== RUN   TestInit_clientTLS
[mysql] 2020/06/12 15:16:30 packets.go:36: unexpected EOF
[mysql] 2020/06/12 15:16:30 packets.go:36: unexpected EOF
[mysql] 2020/06/12 15:16:30 packets.go:36: unexpected EOF
[mysql] 2020/06/12 15:16:31 packets.go:36: unexpected EOF
[mysql] 2020/06/12 15:16:31 packets.go:36: unexpected EOF
[mysql] 2020/06/12 15:16:31 packets.go:36: unexpected EOF
[mysql] 2020/06/12 15:16:32 packets.go:36: unexpected EOF
[mysql] 2020/06/12 15:16:32 packets.go:36: unexpected EOF
[mysql] 2020/06/12 15:16:32 packets.go:36: unexpected EOF
[mysql] 2020/06/12 15:16:33 packets.go:36: unexpected EOF
[mysql] 2020/06/12 15:16:33 packets.go:36: unexpected EOF
[mysql] 2020/06/12 15:16:33 packets.go:36: unexpected EOF
[mysql] 2020/06/12 15:16:35 packets.go:36: unexpected EOF
[mysql] 2020/06/12 15:16:35 packets.go:36: unexpected EOF
[mysql] 2020/06/12 15:16:35 packets.go:36: unexpected EOF
[mysql] 2020/06/12 15:16:37 packets.go:36: unexpected EOF
[mysql] 2020/06/12 15:16:37 packets.go:36: unexpected EOF
[mysql] 2020/06/12 15:16:37 packets.go:36: unexpected EOF
--- PASS: TestInit_clientTLS (12.43s)
PASS
ok  	github.com/hashicorp/vault/plugins/database/mysql	12.722s

[pcman312]

Looks great overall! There's a handful of small things. You should also run gofmt on the package as there are a few small formatting issues.

[catsby]

Style nit: If we're only evaluating a local variable, it's common to see these statements on one line like so:

if err := opt(&builder); err != nil {
  // reference err here
}

[Valarissa]

Interesting style, noted. Used elsewhere, unsure if I changed this one.

[pacificandreascc]

+func (c *mySQLConnectionProducer) Connection(ctx context.Context) (interface{}, error) { + if !c.Initialized { + return nil, connutil.ErrNotInitialized + } + + // If we already have a DB, test it and return + if c.db != nil { + if err := c.db.PingContext(ctx); err == nil { + return c.db, nil + } + // If the ping was unsuccessful, close it and ignore errors as we'll be + // reestablishing anyways + c.db.Close() + } + + connURL, err := c.finalizeConnectionURL()