FR: allow to split wsrep_provider_options across multiple option file lines

With e.g. optimizer_switch it is possible to change different parameters in that variable on separate option file lines, without overwriting previous changes. E.g.

[mysqld]

optimizer_switch=rowid_filter=off

optimizer_switch=condition_pushdown_from_having=off

changes both the rowid_filter and condition_pushdown_from_having settings to OFF.

With wsrep_provider_options on the other hand each assignments voids the previous one, so e.g.

[mysqld]

wsrep_provider_options=gcs.fc_factor=0.5

wsrep_provider_options=gcs.fc_debug=1

only changes the debug setting, not the factor one.

This makes setting different options rather inconvenient.

For one the assignment line may become rather long when multiple provider options are changed.

Also it is not possible to split settings across multiple option files, e.g. it is not possible this way to put all TLS related settings into a separate option file of its own if both socket options, and other provider options need to be changed.