Check DNS resolution in nodepool ready script

We see occasional name resolution failures on our tests slaves. Before
marking any of these nodes as ready in nodepool check that we can
resolve git.openstack.org and the local pypi mirror on that node.

This should help in preventing tests from failing with these errors by
catching them early and help in debugging by centralizing all occurences
of these failures in the nodepool logs.

Change-Id: I48ec87141687c1f6cd53eaada350fc5ce52cdef4
Merge "Check DNS resolution in nodepool ready script"