This is implemented by defining ${DOMAINS_TXT} in config.sh. If not
set in config.sh, it defaults to the previously (hard-coded) location,
which is ${BASEDIR}/domains.txt.
This adds support for a new config variable CONFIG_D in the main configuration
file. Setting a path to a directoy for CONFIG_D advises letsencrypt.sh to
source any additional files found in the specified CONFIG_D directory, that
have a '.sh' ending.
By default CONFIG_D is not set, meaning letsencrypt.sh does not source any
additional configuration files.
If BASEDIR is set in config.sh, this does not modify the (default) values
of either PRIVATE_KEY, WELLKNOWN or LOCKFILE. One needs to manually specify
those three config variables in config.sh, if a different BASEDIR
should be used by letsencrypt.sh.
This commit makes letsencrypt.sh fully aware of a configured BASEDIR.
Daniel Beyer