docs: documentation for collect during INSTALL

* Adding the inveniomanage collect step to the INSTALL documentation as
  it's now required.

* Renaming INSTALL to INSTALL.rst (See #161)

Signed-off-by: Yoan Blanc <yoan.blanc@cern.ch>
script: registry usage for managers

Signed-off-by: Jiri Kuncar <jiri.kuncar@cern.ch>
script: refactoring of manager loading

* NOTE enables `inveniomanage clean` command.

Signed-off-by: Jiri Kuncar <jiri.kuncar@cern.ch>