Attempt to use virtualenv for bootstrapping

To avoid anvils dependencies causing conflicts with the
prepared, packaged, and installed openstack dependencies
use a virtualenv to install said dependencies and then
use the python that exists in that environment to run anvil
instead of the system python.

Change-Id: I6bb7851baa481b768f92bd828a9cec64e60f190d
Fix missing ProcessExecutionError stdout

- Also introduces a simple context manager to do
  the reraising of an exception after doing some
  other processing

Fixes bug 1286264

Change-Id: I8dded139164074c60c70ed751ca3b83287df6f8d
Multipip code cleanup

- Remove the usage of pip to find locally installed packages
  which does not appear to be used (or really that useful) due
  to how anvil is actually expected to create all the needed
  packages for a complete system.
- Remove the usage of global variables for requirements gathered
  and just create those variables and return/pass them around as
  needed instead.
- Remove sys.exit being used in the middle of functions.

Change-Id: Ia95e7b41ba60b7bb2679d523679f80f9dfef129e
Remove post-bootstrap handling

Since we don't build RPMs on bootstrap any more, we can remove
some code that was used to put them into common repos.

Change-Id: Ifc6669045f861301aed42a75ae9d2a883a0f5fa2
Merge "Fix missing ProcessExecutionError stdout"
Merge "Multipip code cleanup"
Merge "Remove post-bootstrap handling"