pip: Make runas and user work interchangeably

The pip state uses "user", while the module uses "runas".  This needs to
be unified in 0.17.0, but until then this commit makes these arguments
interchangeable.
Add proper deprecation warning for runas arg in pip module

Also remove the ability to use runas interchangeably in the state. If
we're going to deprecate it, no need to let people get used to using
runas in their states.
use salt.utils.warn_until for deprecation warning
remove unnecessary **kwargs
Merge pull request #6339 from terminalmage/quickfix

pip: deprecate runas param