Fix the windows `activate_bin` reference, it was, wrongly `pip_bin`.

* Removed unused code.
* Double to single quotes.
`salt.modules.pip.install()` is now aware of multiple mirrors. Fixes #5940.
Gather the command options as a list as opposed to string concatenation.
Fix unit tests to expect the new, single quotes and less wasted white-space, output.
Add a mocked unit testcase to test for multiple mirrors support. Refs #5940.
Allow packages and editable's to be provided. Multiple editable's can also be provided.
We don't need to pass `-u` when we're only passing unittests to `-n`.
Allow passing an actual list of packages to `salt.modules.pip.install()`.
Add multiple editable's and pkgs+editable's test cases.
Added integration test cases for multiple editables and pkgs for `salt.modules.pip.install()`
Gather the uninstall command options as a list as opposed to string concatenation.
Switch `Exception` for `CommandExecutionError`.
Finish what was started on f78fca32
`pip` allows multiple `--find-links` to be passed. Added mocked tests too.
Test for the proper exception, remove stray character left behind.
Switch `assertRaisesWithMessage` with `assertRaises`. The first was not working as supposed.
Merge pull request #5949 from s0undt3ch/issues/5940-multiple-pip-mirrors

Handle multiple mirrors, editable(s) and pkgs on `salt.modules.pip.install()`