Whitelist certain MASTER_SITES in portlint

[Steven Kreuzer]

I would like to propose the following change to portlint to silence  
the warning when only one MASTER_SITES is defined under certain  
conditions.
It adds a variable that is a list of "cloud" providers such as google  
code that appear as a single site but are, in theory,
massively redundant. If a whitelisted site is set for MASTER_SITES, it  
doesn't throw that warning.

Patch for review: http://exit2shell.com/~skreuzer/patches/portlint.patch

--
Steven Kreuzer
http://www.exit2shell.com/~skreuzer