Vault api code panics (called from nomad 0.8.4)

[groggemans]

Describe the bug
Nomad panics on vault api code.

To Reproduce
Run a nomad 0.8.4 server connected to a vault 0.11.0 cluster/server.

Additional context

WARNING: keyring exists but -encrypt given, using keyring
==> Loaded configuration from /etc/nomad.d/base.hcl, /etc/nomad.d/client.hcl, /etc/nomad.d/server.hcl
==> Starting Nomad agent...
panic: interface conversion: interface {} is nil, not []interface {}

goroutine 67 [running]:
github.com/hashicorp/nomad/vendor/github.com/hashicorp/vault/api.(*Sys).Capabilities(0x13bd8dc8, 0x13b20181, 0x24, 0x13bfd17, 0x15, 0x0, 0x0, 0x0, 0x0, 0x0)
        /opt/gopath/src/github.com/hashicorp/nomad/vendor/github.com/hashicorp/vault/api/sys_capabilities.go:38 +0x4c4
github.com/hashicorp/nomad/vendor/github.com/hashicorp/vault/api.(*Sys).CapabilitiesSelf(0x13bd8dc8, 0x13bfd17, 0x15, 0x0, 0x13784780, 0x76d6a6c8, 0x0, 0x0)
        /opt/gopath/src/github.com/hashicorp/nomad/vendor/github.com/hashicorp/vault/api/sys_capabilities.go:6 +0x3c
github.com/hashicorp/nomad/nomad.(*vaultClient).hasCapability(0x13b821b0, 0x13bfd17, 0x15, 0x1f0d668, 0x2, 0x2, 0x22ac4, 0x13ac0320, 0x10, 0x10, ...)
        /opt/gopath/src/github.com/hashicorp/nomad/nomad/vault.go:753 +0x38
github.com/hashicorp/nomad/nomad.(*vaultClient).validateCapabilities(0x13b821b0, 0x13b1c8a1, 0xd, 0x139c4100, 0x0, 0x0)
        /opt/gopath/src/github.com/hashicorp/nomad/nomad/vault.go:696 +0x68
github.com/hashicorp/nomad/nomad.(*vaultClient).parseSelfToken(0x13b821b0, 0x1, 0x0)
        /opt/gopath/src/github.com/hashicorp/nomad/nomad/vault.go:668 +0x1a0
github.com/hashicorp/nomad/nomad.(*vaultClient).establishConnection(0x13b821b0)
        /opt/gopath/src/github.com/hashicorp/nomad/nomad/vault.go:428 +0x11c
github.com/hashicorp/nomad/nomad.(*vaultClient).(github.com/hashicorp/nomad/nomad.establishConnection)-fm()
        /opt/gopath/src/github.com/hashicorp/nomad/nomad/vault.go:247 +0x1c
github.com/hashicorp/nomad/nomad.wrapNilError.func1(0x0, 0x1399d040)
        /opt/gopath/src/github.com/hashicorp/nomad/nomad/vault.go:1186 +0x1c
github.com/hashicorp/nomad/vendor/gopkg.in/tomb%2ev2.(*Tomb).run(0x139a8e40, 0x1370f718)
        /opt/gopath/src/github.com/hashicorp/nomad/vendor/gopkg.in/tomb.v2/tomb.go:153 +0x1c
created by github.com/hashicorp/nomad/vendor/gopkg.in/tomb%2ev2.(*Tomb).Go
        /opt/gopath/src/github.com/hashicorp/nomad/vendor/gopkg.in/tomb.v2/tomb.go:149 +0xa4

The bug seems to be in the vault code, so reporting the issue here.

Also reported at the nomad project (hashicorp/nomad#4633)

[jefferai]

Fixed by #5244