Remove provider storage from tools search path

Also, register OpenStack keystone search path for tools
and fix getKeystoneImageSource so it doesn't error if
the service isn't defined in keystone.

To minimise the impact in the short term, I've added a
"DefaultToolsStorage" field to JujuConnSuite which is
an environs.Storage that overrides the default tools
location (i.e. streams.canonical.com/juju/tools). Tests
that previously uploaded tools to provider storage now
mostly use this.