implement appRole login

[fcgravalos]

Fixes #27

• Implement AppRole auth method. This will make secrets-manager more resilient to network issues or sealing issues.
• secrets-manager will continue to renew its token, but in case there's a failure to fetch such token it will try login again and get a new one.

cc @rvojcik

[fcgravalos]

CircleCI build failing beacuse (I think) mock switched to go modules, using golang.org/x/tools/packages

https://github.com/golang/mock/commit/79d8603a659da203b11a30b9faeb999604924d86

[fcgravalos]

I fixed the above issue, but now hitting this weirdo:

mockgen -package mocks \
    -destination mocks/kubernetes.go \
    -source kubernetes/kubernetes.go \
    -mock_names Client=MockKubernetesClient
2019/05/19 20:56:03 Loading input failed: unsupported version of go: exit status 2: flag provided but not defined: -compiled
usage: list [-e] [-f format] [-json] [build flags] [packages]
Run 'go help list' for details.
Makefile:54: recipe for target 'mocks' failed
make: *** [mocks] Error 1
Exited with code 2

Locally works, though :)

[codecov-io]

Codecov Report

Merging #29 into master will increase coverage by 0.7%. The diff coverage is 91.17%.

@@            Coverage Diff            @@
##           master      #29     +/-   ##
=========================================
+ Coverage   83.79%   84.49%   +0.7%     
=========================================
  Files          11       11             
  Lines         432      458     +26     
=========================================
+ Hits          362      387     +25     
- Misses         55       56      +1     
  Partials       15       15

Impacted Files	Coverage Δ
backend/backend.go	100% <ø> (ø)	:arrow_up:
backend/vault_metrics.go	100% <100%> (ø)	:arrow_up:
backend/vault.go	68.84% <88.46%> (+2.17%)	:arrow_up:
backend/vault_engine.go	87.5% <0%> (+12.5%)	:arrow_up:

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[fcgravalos]

Had to update circleci golang version, that fixed it :)