Standardize on X-Project-Id over X-Tenant-Id.

Both X-Project-Id and X-Tenant-Id headers are set to the
same value by the keystoneclient authtoken middleware.

However the latter is deprecated, so instead of referring to
a mix of the two headers, we standardize on X-Project-Id.

Change-Id: I0fcdc1944ea12d0e997627c5740b57a0bd58dbe8
Merge "Standardize on X-Project-Id over X-Tenant-Id."