Add a configuration variable to optionally allow people to specify their own Octopus.Tentacle.msi file

[robdmoore]

I think the current one (latest after #10 is merged) should be the default.

In case the latest one isn't working for some reason people should be able to choose a tentacle version that does work. Alternatively, if for security or speed reasons someone wanted to host an MSI somewhere in their own control they should also be able to do that.

In both of those circumstances a service configuration variable that is left blank for the default, but can be specified to override it would be cool.