Remove Slicehost driver.

Slicehost API has been shut down in 2012 so it makes no sense to keep
this driver.
Modify drivers for public cloud providers which use HTTP Basic
authentication to not allow insecure connections by default (secure kwarg
being set to False).

This way credentials can't accidentaly be sent in plain text over the
write.

Affected drivers: Bluebox, Joyent, NephoScale, OpSource, VPSNet