Panic with invalid configuration file

[vflaux]

Panic when the configuration file has no "providers".
Encountered this after upgrading osprey from v1 to v2 as the configuration format changed.

Steps to reproduce:

$ > ~/.osprey/config
$ osprey user
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x91363b]

goroutine 1 [running]:
github.com/sky-uk/osprey/client.LoadConfig(0xc0000365c0, 0x1b, 0x8a5627, 0xae4920, 0xc00016b230)
	/home/travis/gopath/src/github.com/sky-uk/osprey/client/config.go:86 +0x1db
github.com/sky-uk/osprey/cmd.user(0x118fc60, 0x11b6a50, 0x0, 0x0)
	/home/travis/gopath/src/github.com/sky-uk/osprey/cmd/user.go:34 +0x4e
github.com/spf13/cobra.(*Command).execute(0x118fc60, 0x11b6a50, 0x0, 0x0, 0x118fc60, 0x11b6a50)
	/home/travis/gopath/pkg/mod/github.com/spf13/cobra@v0.0.5/command.go:830 +0x2aa
github.com/spf13/cobra.(*Command).ExecuteC(0x1190b60, 0x43d25a, 0x11437a0, 0xc000000180)
	/home/travis/gopath/pkg/mod/github.com/spf13/cobra@v0.0.5/command.go:914 +0x2fb
github.com/spf13/cobra.(*Command).Execute(...)
	/home/travis/gopath/pkg/mod/github.com/spf13/cobra@v0.0.5/command.go:864
github.com/sky-uk/osprey/cmd.Execute()
	/home/travis/gopath/src/github.com/sky-uk/osprey/cmd/root.go:16 +0x31
main.main()
	/home/travis/gopath/src/github.com/sky-uk/osprey/main.go:6 +0x20