[SPARK-23644][CORE][UI][BACKPORT-2.3] Use absolute path for REST call in SHS

[mgaido91]

What changes were proposed in this pull request?

SHS is using a relative path for the REST API call to get the list of the application is a relative path call. In case of the SHS being consumed through a proxy, it can be an issue if the path doesn't end with a "/".

Therefore, we should use an absolute path for the REST call as it is done for all the other resources.

How was this patch tested?

manual tests
Before the change:

After the change:

[mgaido91]

cc @jerryshao

[SparkQA]

Test build #88308 has finished for PR 20847 at commit f130a36.

• This patch fails Spark unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[mgaido91]

retest this please

[SparkQA]

Test build #88315 has finished for PR 20847 at commit f130a36.

• This patch fails Spark unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[kiszk]

retest this please

[SparkQA]

Test build #88322 has finished for PR 20847 at commit f130a36.

• This patch fails from timeout after a configured wait of `250m`.
• This patch merges cleanly.
• This patch adds no public classes.

[dongjoon-hyun]

Retest this please.

[SparkQA]

Test build #88335 has finished for PR 20847 at commit f130a36.

• This patch fails Spark unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[mgaido91]

most of the UTs are failing consistently and of course UTs failures are unrelated. Since this is on branch 2.3, shall we open a JIRA for tracking this?

[jerryshao]

Jenkins, retest this please.

[jerryshao]

@mgaido91 would you please create a JIRA to track all the UT failures. Hopes someone familiar with that part could fix them.

[SparkQA]

Test build #88364 has finished for PR 20847 at commit f130a36.

• This patch fails due to an unknown error code, -9.
• This patch merges cleanly.
• This patch adds no public classes.

[jerryshao]

Jenkins, retest this please.

[SparkQA]

Test build #88371 has finished for PR 20847 at commit f130a36.

• This patch fails Spark unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[dongjoon-hyun]

The ML bug is fixed. I'll retrigger this.

[dongjoon-hyun]

Retest this please.

[SparkQA]

Test build #88382 has finished for PR 20847 at commit f130a36.

• This patch fails Spark unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[dongjoon-hyun]

Retest this please.

[SparkQA]

Test build #88387 has finished for PR 20847 at commit f130a36.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[jerryshao]

Thanks, merging to branch 2.3.

[jerryshao]

@mgaido91 this is already merged to branch 2.3. Please close this PR if it is not closed automatically.