Use `organisation` consistently in CTA software

Here is an illustration of the valid organi[s|z]ation combination that does not require any comment:

cta-admin virtualorganization add --virtualorganisation repack --readmaxdrives 0 --writemaxdrives 0 --diskinstance eosctarepack \
 --isrepackvo true --comment "Tape Media Repack framework"

The cta-admin command is virtualorgani**z**ation and the long option name is virtualorgani**s**ation.

We must make sure that we consistently use virtualorganisation in the full software stack, man pages, documentations...