[12.0 stable] Wait for wwan to apply latest config before testing connectivity

[milan-zededa]

When cellular network config changes, it may take few seconds for the wwan microservice and the modem to apply the new config. NIM microservice should wait for the wwan config to be fully applied when cellular connectivity is used for management and ethernet is not available. Otherwise, it may determine the connectivity as not working and failover to the previous config too fast, not giving the modem the chance to connect with the latest config.

Also fixed in this commit is the Equal() method for CellularAccessPoint. It used to compare also timestamp inside ErrorAndTime which may change even if there is no actual config change. As a result, wwan microservice would trigger re-connection even when there was no config change to apply. This fix is in pillar and will be propagated into wwan/mmagent in the next commit (with "go get") once this one is merged.

(cherry picked from commit f208c6763fad4609cdbdc5c76ff846b3cec1da4a)