Handle exceptions on create_dhcp_port

If a network/subnet is deleted while creating the dhcp
port, the agent will detect a conflict on state of the
network and deal with it accordingly.

A concurrent delete may manifest itself via a number
of exceptions, IPAddressGenerationFailure amongst others,
hence the refactoring of the error handling logic into its
own utility method.

Partial-bug: #1253344
Related-bug: #1243726
Partial-rally-bug: DE1004
Upstream-Review: https://review.openstack.org/#/c/57812/
Backported-from: icehouse
Change-Id: I442beb5f82f3db8786eea53926903ef0ba0efbf1
Handle failures on update_dhcp_port

Ensure exceptions due to conflicting
state of network or subnet resources
are dealt with by the dhcp agent.

Closes-bug:  #1253344
Related-bug: #1243726
Partial-rally-bug: DE1004
Upstream-Review: https://review.openstack.org/#/c/59664/
Backported-from: icehouse
Change-Id: I4fd51442c034fabc91d5a3f065f4df98f5fad35b
Merge pull request #48 from marcsolanas/DE1004/59664

De1004/59664