Add saharaclient to global requirements

The python-saharaclient 0.6.0 release is a fully renamed version of ex.
savanna client with backward compat for both python bindings and cli,
so, we're planning to switch external projects requirements to the
saharaclient package imports and sahara cli command usage and then remove
python-savannaclient from the global requirements.

Change-Id: I97ea6560580931452c8739e4f6961412c873dac1
Merge "Add saharaclient to global requirements"