test(git_push): add concurrent push specs

[mboersma]

$ FOCUS_TEST="git push deis master" DEIS_CONTROLLER_URL=your.workflow.host make test-integration
...
Ran 6 of 179 Specs in 364.039 seconds
SUCCESS! -- 6 Passed | 0 Failed | 0 Pending | 173 Skipped 

(Add -v in the Makefile TEST_OPTS to see hilariously interleaved buildpack/dockerfile output.)

Refs #224.

cc: @vdice.

TODO:

• curl app endpoints in different app specs
• loop until success in same app specs
• use filepath for portability

[deis-bot]

@krancour is a potential reviewer of this pull request based on my analysis of git blame information. Thanks @mboersma!

[vdice]

@mboersma

loop until success in same app specs

Is covered in #237 if we want to pull that logic in...