Use Fabric for some of these commands

[mahmoudimus]

https://github.com/balanced/bob/blob/master/bob/builders/__init__.py#L17-L32

[mjallday]

ref https://hynek.me/articles/python-app-deployment-with-native-packages/ which uses fabric entirely

[mjallday]

think we can do

from fabric.api import local


class FabricBuilderMixin(object):

    cmd_executor = local

    def run_command(cmd):
        self.cmd_executor(cmd)

which makes it easy to swap out the cmd_executor with the current implementation (popen) and also makes it easy to use fabric's remote executor if we ever decided to move to using a remote build slave.