HBASE-23312 HBase Thrift SPNEGO configs (HBASE-19852) should be backwards compatible

[risdenk]

HBase Thrift SPNEGO configs should not be required.
The hbase.thrift.spnego.keytab.file and
hbase.thrift.spnego.principal configs should fall
back to the hbase.thrift.keytab.file and
hbase.thrift.kerberos.principal configs. This will
avoid any issues during upgrades.

[risdenk]

@joshelser PR for branch 2.1 from PR #850. I'm still running through all the tests locally, but the affected tests all looked good.

[Apache-HBase]

🎊 +1 overall

Vote	Subsystem	Runtime	Comment
+0 🆗	reexec	1m 48s	Docker mode activated.
		_ Prechecks _
+1 💚	dupname	0m 0s	No case conflicting files found.
+1 💚	hbaseanti	0m 0s	Patch does not have any anti-patterns.
+1 💚	@author	0m 0s	The patch does not contain any @author tags.
+1 💚	test4tests	0m 0s	The patch appears to include 2 new or modified test files.
		_ branch-2.1 Compile Tests _
+1 💚	mvninstall	6m 3s	branch-2.1 passed
+1 💚	compile	0m 34s	branch-2.1 passed
+1 💚	checkstyle	0m 29s	branch-2.1 passed
+1 💚	shadedjars	4m 39s	branch has no errors when building our shaded downstream artifacts.
+1 💚	javadoc	0m 32s	branch-2.1 passed
+0 🆗	spotbugs	1m 40s	Used deprecated FindBugs config; considering switching to SpotBugs.
+1 💚	findbugs	1m 38s	branch-2.1 passed
		_ Patch Compile Tests _
+1 💚	mvninstall	5m 32s	the patch passed
+1 💚	compile	0m 37s	the patch passed
+1 💚	javac	0m 37s	the patch passed
+1 💚	checkstyle	0m 29s	the patch passed
+1 💚	whitespace	0m 0s	The patch has no whitespace issues.
+1 💚	shadedjars	4m 32s	patch has no errors when building our shaded downstream artifacts.
+1 💚	hadoopcheck	25m 6s	Patch does not cause any errors with Hadoop 2.7.7 2.8.5 or 3.0.3 3.1.2.
+1 💚	javadoc	0m 31s	the patch passed
+1 💚	findbugs	1m 37s	the patch passed
		_ Other Tests _
+1 💚	unit	2m 36s	hbase-thrift in the patch passed.
+1 💚	asflicense	0m 16s	The patch does not generate ASF License warnings.
		64m 25s

Subsystem	Report/Notes
Docker	Client=19.03.5 Server=19.03.5 base: https://builds.apache.org/job/HBase-PreCommit-GitHub-PR/job/PR-878/1/artifact/out/Dockerfile
GITHUB PR	#878
Optional Tests	dupname asflicense javac javadoc unit spotbugs findbugs shadedjars hadoopcheck hbaseanti checkstyle compile
uname	Linux 21bce4f5c34c 4.15.0-66-generic #75-Ubuntu SMP Tue Oct 1 05:24:09 UTC 2019 x86_64 GNU/Linux
Build tool	maven
Personality	/home/jenkins/jenkins-slave/workspace/HBase-PreCommit-GitHub-PR_PR-878/out/precommit/personality/provided.sh
git revision	branch-2.1 / fde8128
Default Java	1.8.0_181
Test Results	https://builds.apache.org/job/HBase-PreCommit-GitHub-PR/job/PR-878/1/testReport/
Max. process+thread count	1887 (vs. ulimit of 10000)
modules	C: hbase-thrift U: hbase-thrift
Console output	https://builds.apache.org/job/HBase-PreCommit-GitHub-PR/job/PR-878/1/console
versions	git=2.11.0 maven=2018-06-17T18:33:14Z) findbugs=3.1.11
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[risdenk]

@joshelser should be ready for review :)

[joshelser]

LGTM!