python: Replace async with wait
    
    The async argument to add_lockspace() and rem_lockspace() is not valid
    syntax in python 3.7, since async is a keyword now.
    
    Replace async=False with wait=True, already used in other sanlock
    methods.
    
    This change breaks compatibility with existing python 2 code, but we
    plan to ship sanlock 3.8 with python 3 support only on Fedora 30 and
    EL 8.1 which do not support python 2, so this does not break existing
    users.
    
    Python code that wants to support both sanlock 3.7 and 3.8 will have to
    change the behaviour based on python version.
    
    Signed-off-by: Nir Soffer <nsoffer@redhat.com>