Update to work with new one.com procedure

Update to work with new one.com procedure

Since some Months, its no longer possible to add TXT Records with the Name "_acme-challenge" to the base domain.
To override the fallback value, you must use a CNAME and proxy it.
For example.
CNAME _acme-challenge.yourdomain.com => proxy_acme-challenge.yourdomain.com
The TXT Records have to be created on proxy_acme-challenge.yourdomain.com

Since the default CNAME TTL is 3600 seconds, it is recommended to leave the CNAME record.
But if you would like to use the build-in SSL (for your web-site etc.) from one.com, you have to delete the record.

A new variable "ONECOM_KeepCnameProxy" you can set in the account.conf is used to keep the CNAME record.
By default the CNAME record will be removed.

For ex.: SAVED_ONECOM_KeepCnameProxy='1'
to keep the CNAME Record and speedup the process.