Implement retry on NodeLocked exceptions

This adds the capability to retry node locking when a node is already
locked. Two conductor configuration variables are added to control this
behavior, which can be totally disabled by setting the value for the
node_locked_retry_attempts variable to zero.

A new unit test is added to check that the retry will work after first
getting a NodeLocked exception, then succeeding getting the lock on the
next lock attempt.

Change-Id: I60b099beea1bc3a954a5ab4699e623aaa71ba6c5
Blueprint: add-nodelocked-retry
Merge "Implement retry on NodeLocked exceptions"