Revert to using curl to download tools

aria2 is in universe, which would preclude
juju from entering main. aria2 is also not
available on the test machines, and broke
CI. Revert to using curl, but cycle through
the URLs until one succeeds.

Fixes https://bugs.launchpad.net/juju-core/+bug/1364438
Merge pull request #661 from axw/lp1364438-curl-download-tools

Revert to using curl to download tools

aria2 is in universe, which would preclude
juju from entering main. aria2 is also not
available on the test machines, and broke
CI. Revert to using curl, but cycle through
the URLs until one succeeds.

Fixes https://bugs.launchpad.net/juju-core/+bug/1364438