[Configuration]Support bearer tokens in cluster

[ityuhui]

Load bearer tokens and CA file inside cluster.

/cc @brendandburns

[brendandburns]

Thanks for doing this PR, I added some comments.

[ityuhui]

Thank you @brendandburns

I have updated the code.

[brendandburns]

We should switch to secure_getenv see the recommendations here:

https://www.gnu.org/software/libc/manual/html_node/Environment-Access.html

"General-purpose libraries should always prefer this function over getenv to avoid vulnerabilities if the library is referenced from a SUID/SGID program."

[ityuhui]

It's OK. I have upated it. And I add

#define _GNU_SOURCE

to the source code because it is required by the function secure_getenv (http://man7.org/linux/man-pages/man3/getenv.3.html)

"Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
secure_getenv(): _GNU_SOURCE“

If _GNU_SOURCE is not defined, my example program will core-dump.

141             kubeconfig_env = secure_getenv(ENV_KUBECONFIG);
(gdb)
142             if (kubeconfig_env) {
(gdb) p kubeconfig_env
$1 = 0x0
(gdb) n
145                 homedir_env = secure_getenv(ENV_HOME);
(gdb) n
146                 if (homedir_env) {
(gdb) p homedir_env
$2 = 0xffffffffffffe9d1 <error: Cannot access memory at address 0xffffffffffffe9d1>
(gdb) n
147                     configFileName = calloc(strlen(homedir_env) + strlen(KUBE_CONFIG_DEFAULT_LOCATION) + 1, sizeof(char));
(gdb) s
__strlen_avx2 () at ../sysdeps/x86_64/multiarch/strlen-avx2.S:52
52      ../sysdeps/x86_64/multiarch/strlen-avx2.S: No such file or directory.
(gdb) n
53      in ../sysdeps/x86_64/multiarch/strlen-avx2.S
(gdb) c
Continuing.

Program received signal SIGSEGV, Segmentation fault.
__strlen_avx2 () at ../sysdeps/x86_64/multiarch/strlen-avx2.S:62
62      in ../sysdeps/x86_64/multiarch/strlen-avx2.S

[brendandburns]

Sorry one more comment on secure_getenv vs getenv then I think this is ready to merge.

[ityuhui]

Thank you @brendandburns ! The code is updated.

[brendandburns]

/lgtm /approve

…

--brendan

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