[dnsapi] Support adding / removing multiple TXT values for Gandi

[dnsapi] Support adding / removing multiple TXT values for Gandi

Gandi supports setting multiple entries by setting multiple array items
for the rrset_values field in their API. Modify the dns_gandi_livedns.sh
script so that it checks for existing entries, appends new ones if
needed, and removes existing ones individually. This enabled wildcard
certificate support on Gandi.

Fixes the dns_gandi_livedns part of #1261.

Tested for creating a multidomain, multiple wild-card certificate on
Gandi and using a test script executing only the dns_gandi_livedns_add
and dns_gandi_livedns_rm functions.