Remove silly (and thankfully unused) environ in test suite.
Query simplestreams for matching images.
- Escape dollar and dot in and out of mongodb
- Copy values before modifying it.
[r=sidnei],[bug=1205112] Escape dollar and dot in and out of MongoDB

Both dollar sign and dot are not allowed as keys. In fact, dot creates a
sub-document of some sort, which breaks compatibility with pyjuju and causes a
panic (LP:1205112).

The developer manual suggests escaping those with their unicode full width
equivalents:

  http://docs.mongodb.org/manual/faq/developers/#faq-dollar-sign-escaping
[r=jtv] Simplestreams query for Azure.

This adds to Azure the ability to query simplestreams for suitable OS images.
It's not hooked up into combined instance-type and image selection; that's for
a later branch.

Testing this was tough. It's very hard to figure out why this isn't working
when it isn't — all you get is an empty list of matches. I would have done
more unit-testing, but I'm exhausted.

There's also a little drive-by cleanup: the base test suite for Azure created
an Environ for tests to use. This is bad practice, but luckily no tests were
making use of it. I removed it before anyone gets it into their heads to make
use of the thing.