Allow invalidate of previous credentials for a user AS

[juagargi]

Once a user AS is configured, we obtain a host id and secret that will be valid for the existence of this AS, as well as a VPN client certificate.
It maybe useful to be able to invalidate them for some reasons:

• The credentials are used somewhere else, and we want to invalidate other "usages" but this.
• The credentials were compromised.

This by itself is not enough to avoid the user AS to connect to the AP.
Ideally we would be able to prevent existing user ASes configured before selecting "Invalidate Previous Credentials" from even connecting to the AP.

[FR4NK-W]

"Invalidating" an AS is done by deleting it from the coordinator (and creating a new one if needed)
Otherwise revocation needs to be done properly.