Add an importutils function for try_import

[Dan Prince]

We do a lot of:

try:
    import foo
except...

in our modules.

Lets implement a simple function in our importutils codebase to handle this a bit more cleanly.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/19646

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/19646
Committed: http://github.com/openstack/oslo-incubator/commit/33b12d3c18a35f85785227267347f9ac7f7273a4
Submitter: Jenkins
Branch: master

commit 33b12d3c18a35f85785227267347f9ac7f7273a4
Author: Dan Prince <email address hidden>
Date: Mon Jan 14 12:54:43 2013 -0500

    Implement importutils.try_import.

    Adds a new (simple) importutils.try_import function which
    can return a module name or a default (by default this is None).

    This should help clean up some of our

    try:
        import foo
    except...

    blocks in modules.

    This commit also drops the dependency on python-extras which is
    no longer needed.

    Fixes LP Bug #1099501.

    Change-Id: I8591f79983bdad67a50d1aaae6dce8428dfce084