Consume version 0.19.0 of conjur-authn-k8s-client

This version introduces some changes that we can benefit from, especially these:
- Errors in the certificate injection process on login are now printed to the client logs.
  [cyberark/conjur-authn-k8s-client#/170](cyberark/conjur-authn-k8s-client#170)

go.mod

@@ -5,7 +5,7 @@ go 1.12
 require (
 	github.com/cenkalti/backoff v2.2.1+incompatible
 	github.com/cyberark/conjur-api-go v0.6.0
-	github.com/cyberark/conjur-authn-k8s-client v0.18.1
+	github.com/cyberark/conjur-authn-k8s-client v0.19.0
 	github.com/gogo/protobuf v1.3.1 // indirect
 	github.com/googleapis/gnostic v0.3.1 // indirect
 	github.com/json-iterator/go v1.1.9 // indirect

go.sum

@@ -5,8 +5,7 @@ github.com/cenkalti/backoff v2.2.1+incompatible h1:tNowT99t7UNflLxfYYSlKYsBpXdEe
 github.com/cenkalti/backoff v2.2.1+incompatible/go.mod h1:90ReRw6GdpyfrHakVjL/QHaoyV4aDUVVkXQJJJ3NXXM=
 github.com/cyberark/conjur-api-go v0.6.0 h1:QQYmFRhcCvmtZ9oSRoXCxWb7uRjppfu5lcEwo4HEjtg=
 github.com/cyberark/conjur-api-go v0.6.0/go.mod h1:uM96pLpckwYYAWRSbrsw+TT0y3kg49QCEGpdpa9dJ34=
-github.com/cyberark/conjur-authn-k8s-client v0.18.1 h1:ka397W4Q6AyBxzszvFtuuTtBN6CQ2S5RCMjGsmhonww=
-github.com/cyberark/conjur-authn-k8s-client v0.18.1/go.mod h1:qacUJXCppU1Rg/C+br9B1jBitTq4yG04oc4a+cfI200=
+github.com/cyberark/conjur-authn-k8s-client v0.19.0/go.mod h1:qacUJXCppU1Rg/C+br9B1jBitTq4yG04oc4a+cfI200=
 github.com/davecgh/go-spew v1.1.0/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
 github.com/davecgh/go-spew v1.1.1 h1:vj9j/u1bqnvCEfJOwUhtlOARqs3+rkHYY13jYWTU97c=
 github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=