tenant field in project is not immutable

[mwennrich]

Currently, it is possible to change the tenant of a project, which should not be allowed and leads to strange behaviours:

$ cloudctl project edit cd4eac58-46a5-4a31-b59f-2ec207baa817
(change tenant from fits to brm)

$ cloudctl project ls --id cd4eac58-46a5-4a31-b59f-2ec207baa817
UID                                     TENANT  NAME
cd4eac58-46a5-4a31-b59f-2ec207baa817    ftbrm   mwen

$ cloudctl cluster ls --project cd4eac58-46a5-4a31-b59f-2ec207baa817
UID                                     TENANT  PROJECT                                 NAME    VERSION
ada1bdb0-f51f-4532-9691-c94eaa789218    fits    cd4eac58-46a5-4a31-b59f-2ec207baa817    mwen    1.25.16↑

[majst01]

closed with #116