Implement "exec"

[jodh-intel]

The exec command is blocked on network support. Once networking is available, the function clr_oci_vm_connect() needs to be updated to remove the g_critical ("not implemented yet") call.

[jodh-intel]

Blocked on #38.

[devimc]

I took a look to current implementation of exec and I noticed that it uses ssh to connect to VM, this is a good approach but what will happen with minimal container images (busybox) that has not sshd ?

[gorozco1]

good catch @devimc, I think the ssh daemon will be provided by the mini-os when the clear-container is launched.

[gorozco1]

@devimc has some updates about this issue

[grahamwhaley]

now depends on #302

[dlespiau]

As part of this, we need to document that while the exec will work, the things people may use the exec function for (like manual networking setup) may not necessary work because it will be running inside of the VM.

[dlespiau]

Alright, time to close that one. Bugs found for exec should be opened as new issues (we've started already).