Take default value for --sites-subdir in site install command

[greg-1-anderson]

This will be done only if the URI is a simple identifier (e.g. "dev" or "stage"), or if stripping the http:// off the head of the URI results in the name of a folder in sites that already exists.

Continuation of #2981

[weitzman]

This looks like an improvement. Thanks for looking into this tangled code. I removed it hastily in 20749fb#diff-ca1bf65048418d6ca1d1d82c13f88eb7. Perhaps you could look at that function and see if we are replacing what we need to replace. In particular we should load sites.php and respect it during install and re-install.

[greg-1-anderson]

Yes, I reviewed that function last night, but I missed the support for sites.php. I'm going to merge this now and add support for that in a follow-on pull request.