Add an __str__ method to brokers

A few uses of broker.db_file are in printouts where we do need
them, so the administrator may know what's up. Seems like an easy
way to get rid of those is to make brokers identify themselves
with common __str__. Alternative back-end implementations may
supply something other than a filename here, for example a cluster
name and a volume name.

Note that I'm not sure if correct coercion would occur when
brokers are bounced through dictionaries, hence explicit str().

Change-Id: I329788ebd1fbe39ffadcf9f9d5194a74a88dde58
utf8 encode tempurl key

In tempurl middleware, hmac uses the value of account metadata to
generate HMAC-SHA1 signature and hmac must accept a str-type string, not
a unicode string. The meta dict returned from get_info stroges special
chars as unicode however. So just encode it for tempurl using.

Closes-Bug: #1242644
Change-Id: I4be62eea014a573efc4748470de57dccf00e431d
Merge "Add an __str__ method to brokers"