PT-3583 Use memory repository when no mongo connection for basic auth pligin

[vgarvardt]

What does this PR do?

Basic auth plugin falls back to memory repository when no mongo session avaiable. This is the case when file backend is used for api definitions.

Refs https://github.com/hellofresh/janus/issues/385

[codecov[bot]]

Codecov Report

Merging #386 into master will increase coverage by 0.81%. The diff coverage is 71.42%.

@@            Coverage Diff             @@
##           master     #386      +/-   ##
==========================================
+ Coverage   40.32%   41.13%   +0.81%     
==========================================
  Files          75       75              
  Lines        2624     2662      +38     
==========================================
+ Hits         1058     1095      +37     
  Misses       1506     1506              
- Partials       60       61       +1

Impacted Files	Coverage Δ
pkg/plugin/basic/setup.go	90% <71.42%> (+6.21%)	:arrow_up:
pkg/proxy/balancer/weight.go	91.3% <0%> (-3.15%)	:arrow_down:
pkg/proxy/balancer/rr.go	100% <0%> (ø)	:arrow_up:
pkg/plugin/cb/setup.go	90.9% <0%> (+0.58%)	:arrow_up:

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[bghellofresh]

+1

[RTodorov]

+1