heat common : context should split roles from X-Roles

The auth_token code joins roles with a comma delimiter and stores
the result in the X-Roles header, so we need to split X-Roles back
into a list for the context (otherwise the oslo policy code breaks)

Change-Id: I32bd1d6355b6fe0e16e729f35b1d0b5c61346a70
Signed-off-by: Steven Hardy <shardy@redhat.com>
Merge "heat common : context should split roles from X-Roles"