Define SafeFixture base fixture

Define SafeFixture base fixture

Currenty useFixture(myfixture)[1] ensures to call myfixture.cleanUp only
if myfixture.setUp succeed.

This change defines a workaround to ensure cleanUp call even if setUp
fails until testtools/fixtures support it: SafeFixture[2] which ensures
cleanUp call if setUp fails and replaces fixtures.Fixture use by
SafeFixture. This workaround will be removed when the bug will fixed in
testtools and fixtures[3].

[1] testtools.TestCase.useFixture, fixtures.Fixture.useFixture
[2] neutron.tests.tools
[3] see related bugs

Change-Id: I875934e8dde321a450c83fb95d175affd1f3bb83
Closes-Bug: #1464410
Partial-Bug: #1453888
Related-Bug: #1456353
Related-Bug: #1456370