A charm may request principal to perform a DB migration. The charm
may also optionally flag that interface availability should be
gated on the completion of the requested DB migration.
Change-Id: Id9ed69880f06b835ff2385721d4cb22e47f37c5d
Depends-On: Iaefcb81fff5ed8a9441c93ac4c8bac3fa12eef15
Gate ``available`` state on remote end signalling availability.
Depends-On: I9eb5f8a6039b74288a395a584f844507448cdc23
Change-Id: I44a23549e4830d89ba3182250c40dfe395767fb0
When a subordinate charm doesn't provide an explicit value for the
optional "subordinate_configuration" arg, the error "TypeError: argument
of type 'NoneType' is not iterable" happends. Because the principal
charm expects it as a dict as documented in the interface. So let's use
a empty dict as the fallback value.
Change-Id: Ib8456ab32ec8ee5bee160d0b38a61b7ee43e16ca
Closes-Bug: #1803040