Introduce one step AUTH PLAIN for SMTP.

SMTP authentication using AUTH PLAIN supports two ways to send authentication information to the server,
seems that some servers don't support anymore authentication in two steps:

c: AUTH PLAIN
S: 334
C: encoded username and password
S: 235 2.7.0 Authentication successful

Auth in one step introduced by this commit, reduces communication with the server

c: AUTH PLAIN enconded username and password
S: 235 2.7.0 Authentication successful

Change-Id: I1924af8d7969ce3b3825f2072edabce93c257a34
Signed-off-by: Valerio Valerio <valerio.valerio@jollamobile.com>
Reviewed-by: Matthew Vogt <matthew.vogt@qinetic.com.au>