Support More Complex Cmdline Configuration

Fixes rancher#2755. Allows for quoted arguments with spaces in the kernel cmdline to be parsed properly.

Example: https://play.golang.org/p/GZAUM5cWKJJ

An example command line:

```
$ cat /proc/cmdline
earlyprintk=serial console=ttyS0 rancher.autologin=ttyS0 rancher.defaults.hostname=ros-vm1 rancher.defaults.network.dns.nameservers=[192.168.64.1] rancher.network.interfaces.eth0.dhcp=true rancher.network.interfaces.eth1.dhcp=false rancher.network.interfaces.eth1.address=192.168.99.11/24 rancher.state.dev=LABEL=RANCHER_STATE rancher.state.autoformat=[/dev/vda] rancher.state.formatzero cc.ssh_authorized_keys=['ecdsa-sha2-nistp256 AAAAE2VjZHNhLXNoYTItbmlzdHAyNTYAAAAIbmlzdHAyNTYAAABBBOz8mD0tRrNsHBLHD5jVgmXO26JA7eKFZrj4Ic9KR2y3qXlxU9JCYYn/qDyTCmExt8Rw6SaU/BvgU7WT3Bjsi6c=','ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIJj5mkpBHBBAW5XClcB5aFTWph+VCL7I0W8gm93AT5w4','ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQDKfb0O2qXgIgrtD5Mj7fBYdg4jMrT7wetBbkG2e4maDsRR3AtSYjEB3NeEifM8gdvIf0gYs1BNB/Ar76agaQGeqW+Ewb2LWdypr4Ipw09yWCrC9ttVbCnHuzVLYjML0CNgpjIRC+FC5r1X1gm2LufRN4orZ1NQvNhRRWJVT37vRtHo79TecK0DKQmy87Zpj3cNiI/5iObnTk56pZWpIAEiC5hEVkcVxmdkLJs3YonWVZzmK/Y8uvFtF+GhA6Jcpc38zDQHKsOjFWvj3qbWtVEQteNDxsM2pNeXY5wdrhRn4YSdKme9Cm7CdAogIdAdPtqPIfq/jY0QczS12qFZH7zt']
```

Results in:

```
$ sudo ros config export
rancher:
  defaults:
    hostname: ros-vm1
    network:
      dns:
        nameservers:
        - 192.168.64.1
  environment:
    EXTRA_CMDLINE: /init AAAAC3NzaC1lZDI1NTE5AAAAIJj5mkpBHBBAW5XClcB5aFTWph+VCL7I0W8gm93AT5w4','ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQDKfb0O2qXgIgrtD5Mj7fBYdg4jMrT7wetBbkG2e4maDsRR3AtSYjEB3NeEifM8gdvIf0gYs1BNB/Ar76agaQGeqW+Ewb2LWdypr4Ipw09yWCrC9ttVbCnHuzVLYjML0CNgpjIRC+FC5r1X1gm2LufRN4orZ1NQvNhRRWJVT37vRtHo79TecK0DKQmy87Zpj3cNiI/5iObnTk56pZWpIAEiC5hEVkcVxmdkLJs3YonWVZzmK/Y8uvFtF+GhA6Jcpc38zDQHKsOjFWvj3qbWtVEQteNDxsM2pNeXY5wdrhRn4YSdKme9Cm7CdAogIdAdPtqPIfq/jY0QczS12qFZH7zt']
  network:
    interfaces:
      eth0:
        dhcp: true
      eth1:
        address: 192.168.99.11/24
        dhcp: false
  state:
    autoformat:
    - /dev/vda
    dev: LABEL=RANCHER_STATE
    formatzero: true
ssh_authorized_keys:
- ecdsa-sha2-nistp256 AAAAE2VjZHNhLXNoYTItbmlzdHAyNTYAAAAIbmlzdHAyNTYAAABBBOz8mD0tRrNsHBLHD5jVgmXO26JA7eKFZrj4Ic9KR2y3qXlxU9JCYYn/qDyTCmExt8Rw6SaU/BvgU7WT3Bjsi6c=
- ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIJj5mkpBHBBAW5XClcB5aFTWph+VCL7I0W8gm93AT5w4
- ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQDKfb0O2qXgIgrtD5Mj7fBYdg4jMrT7wetBbkG2e4maDsRR3AtSYjEB3NeEifM8gdvIf0gYs1BNB/Ar76agaQGeqW+Ewb2LWdypr4Ipw09yWCrC9ttVbCnHuzVLYjML0CNgpjIRC+FC5r1X1gm2LufRN4orZ1NQvNhRRWJVT37vRtHo79TecK0DKQmy87Zpj3cNiI/5iObnTk56pZWpIAEiC5hEVkcVxmdkLJs3YonWVZzmK/Y8uvFtF+GhA6Jcpc38zDQHKsOjFWvj3qbWtVEQteNDxsM2pNeXY5wdrhRn4YSdKme9Cm7CdAogIdAdPtqPIfq/jY0QczS12qFZH7zt
```

This is still a bit fragile. For example there can be no space after the commas in the ssh_authorized_keys list. The quoting used above is the only combo I found that works. Also EXTRA_CMDLINE still has part of the last pub key in it.

Based on http://play.golang.org/p/ztqfYiPSlv created by @soheilhy, from a 2014 post https://groups.google.com/forum/#!topic/golang-nuts/pNwqLyfl2co.

config/cmdline/cmdline.go

@@ -3,6 +3,7 @@ package cmdline
 import (
 	"io/ioutil"
 	"strings"
+	"unicode"
 
 	"github.com/rancher/os/pkg/util"
 
@@ -125,11 +126,30 @@ func UnmarshalOrReturnString(value string) (result interface{}) {
 	return
 }
 
+func splitCmdLine(cmdLine string) []string {
+	lastQuote := rune(0)
+	f := func(c rune) bool {
+		switch {
+		case c == lastQuote:
+			lastQuote = rune(0)
+			return false
+		case lastQuote != rune(0):
+			return false
+		case unicode.In(c, unicode.Quotation_Mark):
+			lastQuote = c
+			return false
+		default:
+			return unicode.IsSpace(c)
+		}
+	}
+	return strings.FieldsFunc(cmdLine, f)
+}
+
 func Parse(cmdLine string, parseAll bool) map[interface{}]interface{} {
 	result := map[interface{}]interface{}{}
 
 outer:
-	for _, part := range strings.Split(cmdLine, " ") {
+	for _, part := range splitCmdLine(cmdLine) {
 		if strings.HasPrefix(part, "cc.") {
 			part = part[3:]
 		} else if !strings.HasPrefix(part, "rancher.") {