Change from root to the more general admin.

root is a unix thing and a more general term would be admin. So this
changes the environment variables and arguments from root to admin.
Renamed helper functions return_variable and unset_variable.

I have renamed the helper functions to the shorter read_var and
unset_var.
Renamed function ensure_database_url to set_dokdb_database_url.

Renamed function and made it support a link name of db in addition to
dokdb as a link name.
Renamed common.sh to common.bash