Clean up the postinst script
Clean up the postinst script

- use 'sh' instead of 'bash'
- organize code in functions
- use '[ -n' instead of '[ ! -z'

aleksandrs-ledovskis/omnibus-gitlab,gitlabhq/omnibus-gitlab,copystudy/omnibus-gitlab,gitlabhq/omnibus-gitlab,copystudy/omnibus-gitlab,gitlabhq/omnibus-gitlab,felixbuenemann/omnibus-gitlab,felixbuenemann/omnibus-gitlab,gitlabhq/omnibus-gitlab,felixbuenemann/omnibus-gitlab,aleksandrs-ledovskis/omnibus-gitlab,aleksandrs-ledovskis/omnibus-gitlab,copystudy/omnibus-gitlab