Migration to oslo.utils library

oslo.utils has been released with the modules excutils, importutils,
strutils and timeutils.

Changes done:
- Use the new oslo.utils modules when possible (not updated in the
nova.ironic driver and other oslo.incubator modules)
- importutils.import_module now is importutils.try_import
- Updated requirements.txt with the new library
- strutils.to_bytes now is strutils.string_to_bytes

Once the nova.ironic driver is migrated into nova tree, old references
and libraries can be cleaned (Bug: #1350269)

Closes-Bug: #1353540

Change-Id: Ic0af04ebd07b23eb94df32a6abf0e490d597f32a
Merge "Migration to oslo.utils library"