Change ICS_ROOT from fixed string to on-invoke generation

(Not a task/bug, but as proposal for change)

For now, bootstrap sets a path name of installation into ics_config/ups/ics_config.table as a fixed value (like below), for globally used variable over entire ICS actors and scripts.
> envSet(ICS_MHS_ROOT, "%(ICS_ROOT)")

EUPS sets some environment values like EUPS_DEFAULT_DIR, which are generated and set based on ICS_ROOT if you fleshly installed eups and actors using bootstrap. (If you already have any installation of eups, and points a new install target, these will be different.)
For the production and most of development sites/computers, we will not install eups in a separated way, and mostly we will follow the default way/settings as in the bootstrap.

So, we could be possible to modify a line above as a generated value but not static.

• dependency from actors to ics_config will not change, since we define ics_config as a configuration repository for actors.
• ics_config.table will be tracked by git, it might be difficult to set customized directory in ics_config.table for e.g. 'ICS_MHS_DATA_ROOT', but we can deal with symlink for them (even for now).