Replace try: import with extras.try_import
Replace try: import with extras.try_import

The block form:

try:
    import foo
except:
    foo = None

is confusing to code analysis tools looking for redefinition. It's also a
common enough pattern that someone else has encoded it into a library. Let's
use that instead.

Change-Id: I35a5c7a7e3595a51ba1ef1886c3965f463dba04a

citrix-openstack-build/oslo.service,poznyakandrey/oslo.service,eezhova/oslo.service,openstack/oslo.service