MB-12075: dont crash remote_clusters_info on missing couchApiBase

Our code had assertion that couchApiBaseHTTPS must always exist if
remote cluster's is configured with certificate.

It is usually true that cluster that supports xdcr-over-ssl has all
nodes with couchApiBaseHTTPS. But in this case due to stale cache we
saw one node sending us node info without couchApiBaseHTTPS. Which
caused assertion to crash remote_clusters_info. Which led to
max_restart_intensity_reached.

This commit refactors remote_clusters_info so that extract_node_props
and related functions really error out on missing important fields.

Change-Id: Id62b15ed73ae04b19420bd70b042af3b4049adad
Reviewed-on: http://review.couchbase.org/40953
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>