nodeenv -p should check if node exists in the venv before installing it

[Itxaka]

Calling nodeenv -p several times from a virtualenv will always force install node, even if node is already in there.

It should check for node binaries on the virtualenv and inform the user about it, probably rejecting the reinstall unless the --force flag is passed.

# itxaka @ MacBook-Air-de-Itxaka in ~/projects/horizon on git:master o [10:58:42] tty:s000 L:1 N:76 C:1
$ source .venv/bin/activate
(.venv)
# itxaka @ MacBook-Air-de-Itxaka in ~/projects/horizon on git:master o [10:58:50] tty:s000 L:1 N:77 C:0
$ nodeenv -p --prebuilt
 * Install node (5.9.0)... done.
 * Appending data to /Users/itxaka/projects/horizon/.venv/bin/../bin/activate
(.venv)
# itxaka @ MacBook-Air-de-Itxaka in ~/projects/horizon on git:master o [10:59:12] tty:s000 L:1 N:78 C:0
$ deactivate

# itxaka @ MacBook-Air-de-Itxaka in ~/projects/horizon on git:master o [10:59:18] tty:s000 L:1 N:79 C:0
$ source .venv/bin/activate
(.venv)
# itxaka @ MacBook-Air-de-Itxaka in ~/projects/horizon on git:master o [10:59:20] tty:s000 L:1 N:80 C:0
$ node -v
v5.9.0
(.venv)
# itxaka @ MacBook-Air-de-Itxaka in ~/projects/horizon on git:master o [10:59:23] tty:s000 L:1 N:81 C:0
$ which node
/Users/itxaka/projects/horizon/.venv/bin/node
(.venv)
# itxaka @ MacBook-Air-de-Itxaka in ~/projects/horizon on git:master o [10:59:26] tty:s000 L:1 N:82 C:0
$ nodeenv -p --prebuilt
 * Install node (5.9.0)... done.
 * Appending data to /Users/itxaka/projects/horizon/.venv/bin/../bin/activate