bitbake: xmlrpc: implement check of connection to server

bitbake: xmlrpc: implement check of connection to server

Implemented check_connection function. The purpose of this function
is to check if bitbake server is accessible and functional.
To check this this function tries to connect to bitbake server and
run getVariable command.

This API is going to be used to implement autoloading of bitbake
server.

[YOCTO #5534]

Signed-off-by: Ed Bartosh <ed.bartosh@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>