Add TestFiles Support: Enables additional files to be transfered to the VMs

[landhb]

This PR adds a FileTransfer struct, that contains the user, local path, and remote path for a file transfer:

type FileTransfer struct {
	User    string
	Local   string
	Remote  string
}

And adds a field TestFiles to the Artifact struct, so that multiple files can be defined in the configuration and transferred to test VMs. Example configuration:

# install a utility into the VM
[[test_files]]
user = "root"
local = "./db"
remote = "/usr/bin/db"

# Bring along a test RSA key
[[test_files]]
user = "root"
local = "./db_rsa"
remote = "/tmp/db_rsa"

[[supported_kernels]]
distro_type = "Ubuntu"
distro_release = "16.04"
release_mask = "4[.]4[.]0-70-.*"

[jollheef]

Thank you!

[landhb]

No problem!