Add ssl_passphrase server parameter, which works similarly
to --passout/--passin openssl command line parameters.
Pass phrase value can be formatted as follows.
- pass:password
Provide actual password after the pass: prefix.
- env:var
Obtain the password from the environment variable 'var'a
- file:pathname
Reads the password from the specified file pathname.
Only the first line, up to the newline character, is read from the stream.
If ssl_passphrase was set, SHOW VARIABLE will show "file:", "env:" or
"pass:" (but won't reveal sensitive data)
