Use JSON Web Signature and Encryption (JWS & JWE) between webvirtcloud and gstfsd

[nitmir]

This introduce encryption and signing between webvirtcloud and gstfsd: root password change requests and ssh key change requests are signed and encrypted using a shared secret between webvirtcloud and gstfsd.

On first start, gstfsd generate the shared secret, a JSON Web Key (JWK), and save it in the file /var/lib/gstfsd/SECRET.
You can then add the JWK to computes in webvirtcloud computes panel for webvirtcloud to be able to sign and encrypt request to this compute gstfsd instance. If you try to change a root password without having imported the JWK, an error message tell you how to do so.

This fix #106

[nitmir]

I added 3 commits:

• the first one disable auto-complete on the root password input (so the browser do not give the list of previously entered root password by clicking on the input)
• the second one allow to disable the root password of the VM by settings an empty password
• the third one generate a random salt for root password instead of using always the same salt

[nitmir]

Is there something wrong with this pull requests ?