Use base64 encoding for netconf device passwords

Use base64 encoding for netconf device passwords

Changed password type to binary for encrypted authentication case to
enforce base64 encoding for odl-netconf-device model.

The binary type is in ODL represented by base64 encoding according
to RFC 7950.

This way we can ensure that authentication service is always able
to decrypt such password during connecting process.

We aim to not touch RPC create-device in netconf-node-topology model
thus we are no more reusing credentials from odl-netconf-device
but we create its own which are still using plain password which is
then encoded by RPC logic and translated into new binary format.

JIRA: NETCONF-1115
Change-Id: Ib70c4f69643162b49d8096ec255f3640ebee2e15
Signed-off-by: Oleksandr Zharov <oleksandr.zharov@pantheon.tech>
Signed-off-by: Ivan Hrasko <ivan.hrasko@pantheon.tech>