Allow setting of MySQL bind address in NovaInstance config

Allow override of the MySQL server bind address through the
mysql_server_bind_address puppet variable.  The default will still be
127.0.0.1 (end result: Instance administrators may now set their MySQL
server's bind address in their NovaInstance configuration).

Change from last patchset: Moved changes from
generic::mysql::server to role::labs-mysql-server.  Where it's so much
nicer, too.  Why didn't I think of this before?!

Change-Id: I33cef6cf78df4102d0508df4683e84fcc9579aae
Merge "Allow setting of MySQL bind address in NovaInstance config" into production