Handle IPAddressGenerationFailure during get_dhcp_port

If a network/subnet is deleted while the dhcp agent is trying
to get/create a dhcp port for that network, the exception
will be raised because no IP Allocation Range is available.

However, this particular failure mode causes just noise, because
the dhcp agent can cope with it without problems.

A follow-up patch will deal with the other exception traces during
create_dhcp_port

Partial-bug: #1253344
Partial-rally-bug: DE1004
Upstream-Review: https://review.openstack.org/#/c/57775/
Backported-from: icehouse
Change-Id: I7fe35455ce905daa22ff96367e120864a7d3fb92
Merge pull request #47 from marcsolanas/DE1004/57775

Handle IPAddressGenerationFailure during get_dhcp_port
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