[BEAM-725] Migrate to use the generic Google credentials library

[lukecwik]

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---

Generic library has Apiary and gRPC library support instead of the Apiary only credentials we are currently using.

Dependency diff for modified pom files (old = left, new = right):
beam-sdks-java-core:

8d7
< [INFO] |  \- com.google.auth:google-auth-library-credentials:jar:0.4.0:compile
26c25,26
< [INFO] +- com.google.auth:google-auth-library-oauth2-http:jar:0.4.0:compile

---
> [INFO] +- com.google.auth:google-auth-library-credentials:jar:0.6.0:compile
> [INFO] +- com.google.auth:google-auth-library-oauth2-http:jar:0.6.0:compile
49d48
< [INFO] +- com.google.oauth-client:google-oauth-client-java6:jar:1.22.0:compile
53c52,53
< [INFO] |  \- com.google.api-client:google-api-client-jackson2:jar:1.22.0:compile

---
> [INFO] |  +- com.google.api-client:google-api-client-jackson2:jar:1.22.0:compile
> [INFO] |  \- com.google.oauth-client:google-oauth-client-java6:jar:1.22.0:compile

beam-runners-google-cloud-dataflow-java:

9d8
< [INFO] |  |  \- com.google.auth:google-auth-library-credentials:jar:0.4.0:compile
26d24
< [INFO] |  +- com.google.auth:google-auth-library-oauth2-http:jar:0.4.0:compile
41c39
< [INFO] |  +- com.google.oauth-client:google-oauth-client-java6:jar:1.22.0:compile

---
> [INFO] |  +- com.google.oauth-client:google-oauth-client:jar:1.22.0:compile
52d49
< [INFO] +- com.google.oauth-client:google-oauth-client:jar:1.22.0:compile
61a59,60
> [INFO] +- com.google.auth:google-auth-library-credentials:jar:0.6.0:compile
> [INFO] +- com.google.auth:google-auth-library-oauth2-http:jar:0.6.0:compile
64c63,64
< [INFO] |  \- com.google.api-client:google-api-client-jackson2:jar:1.22.0:compile

---
> [INFO] |  +- com.google.api-client:google-api-client-jackson2:jar:1.22.0:compile
> [INFO] |  \- com.google.oauth-client:google-oauth-client-java6:jar:1.22.0:compile

beam-sdks-java-io-google-cloud-platform:

20d19
< [INFO] |  +- com.google.auth:google-auth-library-oauth2-http:jar:0.4.0:compile
33c32
< [INFO] |  +- com.google.oauth-client:google-oauth-client-java6:jar:1.22.0:compile

---
> [INFO] |  +- com.google.oauth-client:google-oauth-client:jar:1.22.0:compile
44c43,44
< [INFO] |  \- com.google.api-client:google-api-client-jackson2:jar:1.22.0:compile

---
> [INFO] |  +- com.google.api-client:google-api-client-jackson2:jar:1.22.0:compile
> [INFO] |  \- com.google.oauth-client:google-oauth-client-java6:jar:1.22.0:compile
57d56
< [INFO] |  +- com.google.auth:google-auth-library-credentials:jar:0.4.0:compile
69c68,69
< [INFO] +- com.google.oauth-client:google-oauth-client:jar:1.22.0:compile

---
> [INFO] +- com.google.auth:google-auth-library-credentials:jar:0.6.0:compile
> [INFO] +- com.google.auth:google-auth-library-oauth2-http:jar:0.6.0:compile

[lukecwik]

R: @tgroh

[tgroh]

LGTM