use a flag not a subcommand for installing repo files

Because argparser cannot handle optional subcommands and
will error out stating that 'repo' is required.

See Python tracker issue: http://bugs.python.org/issue9253

Signed-off-by: Alfredo Deza <alfredo.deza@inktank.com>
update docs to reflect the flag change

Signed-off-by: Alfredo Deza <alfredo.deza@inktank.com>
add help information for the --repo flag

Signed-off-by: Alfredo Deza <alfredo.deza@inktank.com>
Merge pull request #223 from alfredodeza/8916

use a flag, not a subcommand for installing repo files