Handle SSH keys that have multiple spaces between each marker

Notice what happens when a user adds a key with a space in between:

```
irb(main):004:0> 'ssh-rsa  foobar'.split(/ /)
=> ["ssh-rsa", "", "foobar"]
```

This would cause gitlab-shell to receive a blank argument for the actual
key, leading to users unable to login.

changelogs/unreleased/sh-fix-ssh-keys-with-spaces.yml

@@ -0,0 +1,4 @@
+---
+title: Handle SSH keys that have multiple spaces between each marker
+merge_request:
+author:

lib/gitlab/shell.rb

@@ -35,7 +35,7 @@ module Gitlab
       end
 
       def strip_key(key)
-        key.split(/ /)[0, 2].join(' ')
+        key.split(/[ ]+/)[0, 2].join(' ')
       end
 
       private

spec/lib/gitlab/backend/shell_spec.rb

@@ -69,6 +69,15 @@ describe Gitlab::Shell, lib: true do
         expect(io).to have_received(:puts).with("key-42\tssh-rsa foo")
       end
 
+      it 'handles multiple spaces in the key' do
+        io = spy(:io)
+        adder = described_class.new(io)
+
+        adder.add_key('key-42', "ssh-rsa  foo")
+
+        expect(io).to have_received(:puts).with("key-42\tssh-rsa foo")
+      end
+
       it 'raises an exception if the key contains a tab' do
         expect do
           described_class.new(StringIO.new).add_key('key-42', "ssh-rsa\tfoobar")