Enable the ovsdb-subordinate interface to provide chassis
certificates to principal.
This is useful for related charms providing payloads that
expect direct communication with the OVN databases.
While a principal and subordinate charm executes in the same
environment, the payload usually execute under different service
accounts and as such it is impractical and may be less secure to
attempt to provide direct on-disk file access.
Related-Bug: #1918271
Change-Id: I867817dfa7dd43cdb9035af87cbac3371dff13a5
55e82469e8