Updates config.txt

[jakimowb]

This PR enhances read-config.sh to allow for reading of optional parameters and adds support for 3 new optional parameters in the config.txt

1. read-config.sh can be called like

# syntax: read-config.sh PARAMETER_NAME config.txt [DEFAULT_VALUE]
read-config.sh "MY_PARAMETER" $PATH_CONFIG_FILE "default_value"

If my MY_PARAMETER does not exist, the default value will be used. This allows to define
optional config settings, as in the following:

2. USER_GROUP allows to overwrite the default user and group id that is returned by "$(id -u):$(id -g)", e.g. in

docker run \
--rm \
...
-u "$(id -u):$(id -g)" \
"$IMAGE" \
force-level1-csd \

This allows to create files with user rights different to the UID and GID of the user that started the bash script & docker process. For example, my "standard" uid and gui are: uid=7001(jakimowb) gid=9000(eolab).
Adding USER_GROUP = 7001:9003 to the config.txt ensures that FORCE creates files with uid=7001(jakimowb), gid=9003(datacube), so that all datacube users have access to.

3. WVP_START

Allows to overwrite the d paramter in update-wvdb.sh, i.e. the start date to download WV data for

4. DIR_CREDENTIALS

By default, many bash scripts assume that the credentials are located in $HOME

docker run \
--rm \
...
-v "$HOME:/app/credentials" \
...
"$IMAGE" \
force-level1-csd \

DIR_CREDENTIALS allows to overwrite this default location, e.g.
DIR_CREDENTIALS = /home/myuseraccount/.credentials