NSX: Optionally not enforce nat rule match length check

This patch adds the 'raise_on_len_mismatch' parameter to the
'delete_nat_rules_by_match' function. The plugin then leverages
this parameter for ensuring NAT rules deletion operations
are completed successfully even when duplicate rules are found
or no corresponding rules are found at all.

With this change, the 'remove_router_interface' operation will
correctly complete even in cases when NAT rules in Neutron and
the NSX backend are out of sync.

This patch also changes a check in delete_nat_rules_by_match in
order to make it less expensive.

Closes-Bug: 1328181

Change-Id: I856d67ef5ff6264374cb8f2569668da4c205ad9f
(cherry picked from commit 138d9774000335d66c335e036ea96dac7b4a4799)
Merge "NSX: Optionally not enforce nat rule match length check" into stable/icehouse
Merge branch 'stable/icehouse' of https://github.com/openstack/quantum into stable/icehouse