add --dns-credentials-file option to load credential params from env/ini file

[abdeldjalil-fellah]

add --dns-credentials-file option to load credential params from env/ini file, credentials.ini example:

RFC2136_NAMESERVER=127.0.0.1:53
RFC2136_TSIG_KEY=keyname
RFC2136_TSIG_ALGORITHM=hmac-sha256
RFC2136_TSIG_SECRET=YWJjZGVmZGdoaWprbG1ub3BxcnN0dXZ3eHl6MTIzNDU=