Add SoftLayer DNS as a DomainServiceProvider.
The SoftLayer API is a bit of a mess and treats MX and SRV records
differently. This leads to some replication and custom handling issues
to work around.
In this patch I have to change the SRV test case to be _tcp instead of
_protocol because softlayer requires a "known" protocol which AFAICT is
tcp, udp or tls. I think this will be acceptable in most cases.
Signed-off-by: Jamie Lennox <jamielennox@gmail.com>
