Fix regex in salt.modules.apt._get_upgradable

Third capture group assumes no spaces in the source name, and certain
PPAs (Salt's own being one of them) contain a space. Modified the regex
so that it should work with any apt source. This fixes #3733. Also made
a couple pep8 fixes.
Get environment for the runas user
Add runas to rvm.install
Allow for user installations of rvm
Add missing import
Make a copy of the environment

Without making a copy, the environment will be tainted for subsequent
actions.