Implement heartbeat for FileDriver
Implement heartbeat for FileDriver

As the title said this patch just implement the heartbeat api for
FileDriver.
1. You can delivery the parameter 'timeout' via the connect url like
   this: file:///tmp/test_coord?timeout=10. Otherwise the default timeout
   is 10secs.
2. We can call the function: heartbeat(self) periodic
   to make sure the specific member is alive.
3. When you call get_members(), it will compare the last modify time of
   the file with current time, if the delta_seconds greater than
   the timeout, it will mark the member status is dead and will not return
   the member_id.

Change-Id: I8617e17fc1a74e70d7eb5b6f8e12dc9d9a66c755
Closes-Bug: #1664559

openstack/tooz,openstack/tooz