Fix race conditions running pxe_utils tests in parallel

To avoid the race this patch is mocking the DHCPFactory.provider
attribute instead of mocking a specific provider implementation (Neutron
before). The DHCPFactory is a complex class because it's a singleton,
and by mocking a specific provider and not the top level class tests
could override each other.

Closes-Bug: #1365370
Change-Id: I9b1bd69b73606a4b2665170ae076742c0bac68da
Merge "Fix race conditions running pxe_utils tests in parallel"