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
Replace salt.utils.warn_until

This was added recently and isn't available in the 0.16 branch. Manually
specify the deprecation warning for the purposes of the bugfix release.