Fix panics in rancher token and rancher server current commands

[enrichman]

Fix rancher/rancher#44332

As I've moved in this PR only the fix related to the panics. I'm going to then rebase the other PR.

Bug Description

Some workflows will trigger panics due to assignment to nil maps, or a missing check:

// sc.KubeCredentials is nil -> panics
sc.KubeCredentials[id] = cred

serverName := cf.CurrentServer
// missing check for the existance of the value -> panics
URL := cf.Servers[serverName].URL

To reproduce just run a rancher server current without any configuration:

-> % rancher server current 
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x30 pc=0xa14869]

goroutine 1 [running]:
github.com/rancher/cli/cmd.serverCurrent(0xc000195b80?)
        /go/src/github.com/rancher/cli/cmd/server.go:74 +0xc9
github.com/urfave/cli.HandleAction({0xaa5b00?, 0xc28200?}, 0x7?)
...

or run a rancher login followed by a rancher token:

-> % rancher login https://myrancher.com --name https://myrancher.com --token MYTOKEN
INFO[0000] Saving config to /home/enrico/.rancher/cli2.json

###

-> % rancher token --server https://myrancher.com --user local
https://myrancher.com/v3-public/authProviders 
Enter credentials for localProvider 
 
Enter username: admin 
Enter password: 
panic: assignment to entry in nil map

goroutine 1 [running]:
github.com/rancher/cli/cmd.cacheCredential(0xc000274160, 0xc00032a2d0, {0xc0003326a0, 0x6})
        /go/src/github.com/rancher/cli/cmd/kubectl_token.go:283 +0x11a
github.com/rancher/cli/cmd.runCredential(0xc000274160)
        /go/src/github.com/rancher/cli/cmd/kubectl_token.go:165 +0x4a5
github.com/urfave/cli.HandleAction({0xaa5b00?, 0xc281f0?}, 0xc00011cfc0?)
        /go/pkg/mod/github.com/urfave/cli@v1.22.5/app.go:524 +0x50
github.com/urfave/cli.(*App).RunAsSubcommand(0xc00011cfc0, 0xc000274000)
        /go/pkg/mod/github.com/urfave/cli@v1.22.5/app.go:410 +0xabd
github.com/urfave/cli.Command.startApp({{0xbd2fb4, 0x5}, {0x0, 0x0}, {0x0, 0x0, 0x0}, {0xbf2956, 0x2e}, {0x0, ...}, ...}, ...)
        /go/pkg/mod/github.com/urfave/cli@v1.22.5/command.go:372 +0xb7f
github.com/urfave/cli.Command.Run({{0xbd2fb4, 0x5}, {0x0, 0x0}, {0x0, 0x0, 0x0}, {0xbf2956, 0x2e}, {0x0, ...}, ...}, ...)
        /go/pkg/mod/github.com/urfave/cli@v1.22.5/command.go:102 +0x845
github.com/urfave/cli.(*App).Run(0xc00011ce00, {0xc00004fd00, 0x6, 0x8})
        /go/pkg/mod/github.com/urfave/cli@v1.22.5/app.go:277 +0xb67
main.mainErr()
        /go/src/github.com/rancher/cli/main.go:136 +0x1f5d
main.main()
        /go/src/github.com/rancher/cli/main.go:57 +0x19

[crobby]

lgtm
I know the tests are coming in #347

[andreas-kupries]

looks good to me