fix: double quote tags if they are set

[mdelapenya]

What does this PR do?

It double-quotes the tags in Makefile only when they are set

Why is it important?

There was a corner case when using Make to run all scenarios (not filtering by tag)

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have run the Unit tests for the CLI, and they are passing locally
• I have run the End-2-End tests for the suite I'm working on, and they are passing locally
• I have noticed new Go dependencies (run make notice in the proper directory)

How to test this PR locally

Before this change, this command failed because godog was trying to evaluate "" as a feature path. Now it runs all scenarios (no TAGS set) but those skipped.

 SUITE="fleet" DEVELOPER_MODE=false TIMEOUT_FACTOR=3 LOG_LEVEL=TRACE make -C e2e functional-test

This command runs all scenarios (no TAGS set), including skipped.

 SUITE="fleet" SKIP_SCENARIOS=false DEVELOPER_MODE=false TIMEOUT_FACTOR=3 LOG_LEVEL=TRACE make -C e2e functional-test

This command runs some scenarios (TAGS is set), excluding skipped.

 SUITE="fleet" TAGS="fleet_mode_agent" DEVELOPER_MODE=false TIMEOUT_FACTOR=3 LOG_LEVEL=TRACE make -C e2e functional-test

This command runs some scenarios (TAGS is set), including skipped.

 SUITE="fleet" TAGS="fleet_mode_agent" SKIP_SCENARIOS=false DEVELOPER_MODE=false TIMEOUT_FACTOR=3 LOG_LEVEL=TRACE make -C e2e functional-test

Related issues

• Closes using 'make' to run e2e tests without specifying a TAGS= value results in no tests being run #408

[elasticmachine]

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Build stats

• Build Cause: Pull request #531 updated

• Start Time: 2020-12-04T14:53:25.193+0000

• Duration: 5 min 51 sec

Test stats 🧪

Test	Results
Failed	0
Passed	32
Skipped	13
Total	45

💚 Flaky test report

Tests succeeded.

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Test stats 🧪

Test	Results
Failed	0
Passed	32
Skipped	13
Total	45