Improve iptables_manager _modify_rules() method

As the number of ports per default security group increases, the
number of iptables entries on the Compute Node grows.  Because of
this, there is a gradual increase in the time taken to apply chains
and rules.

Currently we are using list comprehensions to find if a new chain or
rule matches an existing one.  Instead, walk through the list in
reverse to find a matching entry.

Added a new method, _find_last_entry(), to return the entry we are
searching for.

Change-Id: I3585479ffa00be556b8b21dc9dbd6b36ad37f4de
Closes-Bug: #1302272
Related-Bug: #1253993
(cherry picked from commit 0c202ab3e453e38c09f04978e4fce30d6ee6350c)

Closes-rally-bug: DE706