s3:pylibsmb: Make lp a mandatory param for the SMB connection

s3:pylibsmb: Make lp a mandatory param for the SMB connection

Currently establishing the SMB connection relies on having initialized
the global source3 loadparm.

This patch makes the lp param mandatory, so that you always have to pass
the parameter in when establishing the SMB connection.

It also makes the source3 API more consistent with the current source4
API, which will make it easier to replace the source4 version later.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13676

Signed-off-by: Tim Beale <timbeale@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>