Break down _bind_port_if_needed in ML2

Break down _bind_port_if_needed in ML2

Separate the looping and retry logic in _bind_port_if_needed
from the actual binding attempts. This also eliminates the
'while True' loop with a regular for loop counter to make it
a little easier to reason about.

A suggestion to do this came up in a code review for
I437290affd8eb87177d0626bf7935a165859cbdd because the function
was difficult to reason about.

Change-Id: I6171cf39a4562ed1da9467e8e604d4519a813977