openvpn: Support username and password options

Some VPN providers require username and password for client to connect.
This commit adds an option to specify username, password and
cert_password directly in uci config which then gets expanded during
start of openpvn client.

Signed-off-by: Michal Hrusecky <michal.hrusecky@turris.com>

net/openvpn/files/openvpn.config

@@ -9,6 +9,13 @@ config openvpn custom_config
 	# Set to 1 to enable this instance:
 	option enabled 0
 
+	# Credentials to login
+	#option username 'login'
+	#option password 'password'
+
+	# Password for client certificate
+	#option cert_password 'cert_password'
+
 	# Include OpenVPN configuration
 	option config /etc/openvpn/my-vpn.conf