Cope with unsupported kernel

Livepatch now checks for the running kernel and let's the user know if it's unsupported. For example:

```
ubuntu@bionic-livepatch:~$ canonical-livepatch enable foo
2018/05/04 19:55:05 cannot use livepatch: your kernel "4.15.0-1008-kvm" is not eligible for livepatch updates
```

`ua status`, however, doesn't interpret that yet and just says that livepatch is disabled:
```
ubuntu@bionic-livepatch:~$ ua status
esm: disabled (not available)
fips: disabled (not available)
livepatch: disabled
```

Furthermore the MOTD component says that livepatch is available for installation, likely because the status output does not say "not available":
```
 * Canonical Livepatch is available for installation.
   - Reduce system reboots and improve kernel security. Activate at:
     https://ubuntu.com/livepatch
```

`ua enable-livepatch <foo>` also doesn't check and tries to enable it, but it eventually fails:
```
ubuntu@bionic-livepatch:~$ sudo ua enable-livepatch <token>
Enabling Livepatch with the given token, stand by...
2018/05/04 19:57:17 cannot use livepatch: your kernel "4.15.0-1008-kvm" is not eligible for livepatch updates
```

We should:

- fix the MOTD when running an unsupported kernel. I suggest to output nothing.
- don't even try to enable livepatch if we are running an unsupported kernel