Mock os.makdirs to avoid directory creation in unit tests

The directory 'dhcp' under neutron state path should not be
created during unit tests. This patch mocks up calls to
os.makedirs and ensures that methods that rely on it, like
_populate_network_cache, are mocked as well.

Fixes bug #1211952

Change-Id: I83998d4e631b1ebeab90b4c3c0276aa2ffe22c50
Merge "Mock os.makdirs to avoid directory creation in unit tests"