bitbake: xmlrpc: add support for token reusing

bitbake: xmlrpc: add support for token reusing

We add support to specify a connection token in the command line
and in the environment variable BBTOKEN.

When a client registers to a bitbake server, that client
will have exclusive access to the server. The client is identified
by a server-supplied token. If a client terminates, we cannot
reconnect to the server as the token is lost.

This patch adds the capability to specify the connection token
in the command line for xmlrpc clients. This allows us
to have bitbake work as an already-authenticated
client with the server and resume sending commands to a server.

(Bitbake rev: db5390940c0afbcdc9fbcf1225761968ae51d4a7)

Signed-off-by: Alexandru DAMIAN <alexandru.damian@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>