Enable connection to a Remote Cluster for easy development

[bzon]

Propose Change

Enable running fluxd binary locally.

go run cmd/fluxd/main.go \
    --memcached-hostname localhost  \
    --memcached-port 11211 \
    --memcached-service "" \
    --git-url [email protected]:bzon/flux-get-started \
    --k8s-in-cluster=false

This is a continuation of the unmaintained PR #1156 started by @andersjanmyr.

I had to add an implementation of ssh.KeyRing interface because without it, fluxctl -u http://localhost:3030/api/flux [command] makes fluxd panics with the below error:

2019/06/09 17:27:51 http: panic serving [::1]:58686: runtime error: invalid memory address or nil pointer dereference
goroutine 767 [running]:
net/http.(*conn).serve.func1(0xc0006365a0)
	/usr/local/Cellar/go/1.11.5/libexec/src/net/http/server.go:1746 +0xd0
panic(0x2159f60, 0x31a9640)
	/usr/local/Cellar/go/1.11.5/libexec/src/runtime/panic.go:513 +0x1b9
github.com/weaveworks/flux/cluster/kubernetes.(*Cluster).PublicSSHKey(0xc0005180f0, 0x12f6500, 0xc0003f0868, 0x2076740, 0xc000611b84, 0x16, 0x0)
	/Users/jb/Golang/src/github.com/weaveworks/flux/cluster/kubernetes/kubernetes.go:276 +0x3a
github.com/weaveworks/flux/daemon.(*Daemon).GitRepoConfig(0xc00033a120, 0x255a2c0, 0xc0004db770, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, ...)
	/Users/jb/Golang/src/github.com/weaveworks/flux/daemon/daemon.go:610 +0x83
github.com/weaveworks/flux/http/daemon.HTTPServer.GitRepoConfig(0x256c940, 0xc00033a120, 0x254bec0, 0xc0007bfdc0, 0xc000ec8600)
	/Users/jb/Golang/src/github.com/weaveworks/flux/http/daemon/server.go:187 +0x181
github.com/weaveworks/flux/http/daemon.HTTPServer.GitRepoConfig-fm(0x254bec0, 0xc0007bfdc0, 0xc000ec8600)
	/Users/jb/Golang/src/github.com/weaveworks/flux/http/daemon/server.go:59 +0x57

[squaremo]

This looks fine to me -- nicely done.