This patch introduces a new user format to tempauth which starts with
"user64_" and in which the account and user are base64 encoded (sans
padding equal signs):
user64_<account_b64>_<user_b64> = <key> [group] [group] [...] [storage_url]
This allows unambiguous parsing and adds the ability to have a user or
account name containing underscores.
The handling of tempauth configuration is fully backward compatible and
this only adds new, optional functionality.
Also beefed up unit tests to get full coverage of the user parsing code
in tempauth's __init__().
Change-Id: Id06304934ab8055a28921f2e1aa3c58a2036d8f8