Allow user-defined environment variables in machine.ini

[hagertnl]

I propose that we consider allowing a user-defined section of environment variables that can be included in .ini and the environment variables are then set in the environment. Additionally, in job script templates, allow a __custom_machine_env__ or similar replacement which is then replaced by these variables in the job script. Example:

vanilla_machine.ini:

...
[Site-custom]
my_custom_env_1 = abc
my_custom_env_2 = def

Then RGT_MY_CUSTOM_ENV_1=abc and RGT_MY_CUSTOM_ENV_2=def are set in the environment.
Then this line in the job script template:

__custommachine_env__`

Could be replaced by

export RGT_MY_CUSTOM_ENV_1=abc
export RGT_MY_CUSTOM_ENV_2=def

Note that the Bash language is not guaranteed for job scripts, so we'd probably have to do some leg work for that.

Currently, you can hack this together using some obscure harness features and hard-to-read templates, this would just make things much easier and more readable.