Handle vault HTTP redirects

[drmdrew]

Prior to vault 0.6.2, vault standby nodes would redirect HTTP to the active vault node. Current versions of the go http package do not seamlessly handle this redirection properly.

Attempting to use gomplate with pre-0.6.2 vault in HA configuration results in a panic, e.g.

echo -n '{{(datasource "vault" "secret/my_secret").value}}'  | gomplate -d vault="vault:///"
panic: runtime error: invalid memory address or nil pointer dereference [recovered]

[drmdrew]

The relevant portions of a sample panic that occurs against pre-0.6.2 vault HA setup when a redirect from a standby is encountered:

panic: runtime error: invalid memory address or nil pointer dereference [recovered]
        panic: runtime error: invalid memory address or nil pointer dereference [recovered]
        panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x20 pc=0xf0b1a]

goroutine 1 [running]:
panic(0x51b940, 0xc4200161c0)
        /usr/local/Cellar/go/1.7.3/libexec/src/runtime/panic.go:500 +0x1a1
github.com/urfave/cli.HandleAction.func1(0xc4201e5d00)
        /Users/dee/gocode/src/github.com/urfave/cli/app.go:478 +0x247
panic(0x51b940, 0xc4200161c0)
        /usr/local/Cellar/go/1.7.3/libexec/src/runtime/panic.go:458 +0x243
text/template.errRecover(0xc4201e5730)
        /usr/local/Cellar/go/1.7.3/libexec/src/text/template/exec.go:140 +0x2ad
panic(0x51b940, 0xc4200161c0)
        /usr/local/Cellar/go/1.7.3/libexec/src/runtime/panic.go:458 +0x243
github.com/hairyhenderson/gomplate/vault.(*Client).Login(0xc4201d2180, 0x5c3b3e, 0x7fff5fbff61f)
        /Users/dee/gocode/src/github.com/hairyhenderson/gomplate/vault/client.go:59 +0x2a
main.readVault(0xc420074d80, 0xc4201ce040, 0x1, 0x1, 0xc420077d10, 0x1, 0x1246c, 0x7fff5fbff619, 0xc4201564bf)
        /Users/dee/gocode/src/github.com/hairyhenderson/gomplate/data.go:244 +0x19a
main.(*Data).ReadSource(0xc4201b2aa0, 0x0, 0x0, 0xc420074d80, 0xc4201ce040, 0x1, 0x1, 0x507048, 0x9, 0xc4201ce051, ...)
        /Users/dee/gocode/src/github.com/hairyhenderson/gomplate/data.go:175 +0x1ec
main.(*Data).Datasource(0xc4201b2aa0, 0xc4201564bf, 0x5, 0xc4201ce040, 0x1, 0x1, 0x179e5)
        /Users/dee/gocode/src/github.com/hairyhenderson/gomplate/data.go:149 +0xd8
main.(*Data).Datasource-fm(0xc4201564bf, 0x5, 0xc4201ce040, 0x1, 0x1, 0x0)
        /Users/dee/gocode/src/github.com/hairyhenderson/gomplate/main.go:67 +0x5c

[hairyhenderson]

Fixed by #76