pkiz String conversion

When the PKIZ token provider signs a token, force conversion of the
token to a string as the keystone client cms code produces unicode.

Without this change, Keystone errors out when validating tokens.  It
was only visible in tempest tests.

bp: compress-tokens
Change-Id: Ic932240fbbe5bd9da6197b37cf86092bde510f40
Merge "pkiz String conversion"