skipper: set server minimal TLS version (#3282)

The tls-min-version flag specifies "minimal TLS Version to be used in server, proxy and client connections"
but was not used to configure Server TLS.

Follow up on: #1656

Signed-off-by: Alexander Yastrebov <yastrebov.alex@gmail.com>

config/config.go

@@ -732,6 +732,7 @@ func (c *Config) ToOptions() skipper.Options {
 		CertPathTLS:               c.CertPathTLS,
 		KeyPathTLS:                c.KeyPathTLS,
 		TLSClientAuth:             c.TLSClientAuth,
+		TLSMinVersion:             c.getMinTLSVersion(),
 		CipherSuites:              c.filterCipherSuites(),
 		MaxLoopbacks:              c.MaxLoopbacks,
 		DefaultHTTPStatus:         c.DefaultHTTPStatus,