Retry Rackspace LB nodes operations if immutable

During certain operations, Rackspace::Cloud::LoadBalancer may reach
immutable states.  If an operation happens at such times, the entire
stack update will fail.  This patch adds retry logic to wait until LB
leaves the immutable state before proceeding.

Closes-Bug: #1401333
Change-Id: I46692f29b57a8c5db9a2dfab11ac5dc9ad631d45
Merge "Retry Rackspace LB nodes operations if immutable"