Problem: Travis will eventually check out the wrong plugin branches

for pulpcore 3.0.

Solution:
1. Regenerate with latest plugin-template,
2. "repurpose" its var pulpcore_branch as the branch for
pulp_file: master/master & 3.0/0.1
3. Specify the current only branch for pulp-certguard: master
4. Add plugin-template's standard bash settings for robustness
and verbosity to pre_before_install.sh

Also cleanup the effectively empty .travis/post_script.sh

[noissue]

re: #5782
plugin-template needs to support both the pulpcore 3.0 and master branch for plugins
https://pulp.plan.io/issues/5782

.travis/install.sh

@@ -56,7 +56,7 @@ PLUGIN=pulp_file
 if [ -e $TRAVIS_BUILD_DIR/../pulp-certguard ]; then
   PULP_CERTGUARD=./pulp-certguard
 else
-  PULP_CERTGUARD=git+https://github.com/pulp/pulp-certguard.git
+  PULP_CERTGUARD=git+https://github.com/pulp/pulp-certguard.git@master
 fi
 
   cat > vars/vars.yaml << VARSYAML

.travis/pre_before_install.sh

@@ -1,8 +1,15 @@
+#!/usr/bin/env bash
+set -mveuo pipefail
+
 export PULP_FILE_PR_NUMBER=$(echo $COMMIT_MSG | grep -oP 'Required\ PR:\ https\:\/\/github\.com\/pulp\/pulp_file\/pull\/(\d+)' | awk -F'/' '{print $7}')
 export PULP_CERTGUARD_PR_NUMBER=$(echo $COMMIT_MSG | grep -oP 'Required\ PR:\ https\:\/\/github\.com\/pulp\/pulp-certguard\/pull\/(\d+)' | awk -F'/' '{print $7}')
 
+pip install yq
+PULP_FILE_BRANCH=$(yq -r .pulpcore_branch ./template_config.yml)
+echo PULP_FILE_BRANCH="$PULP_FILE_BRANCH"
+
 cd ..
-git clone https://github.com/pulp/pulp_file.git
+git clone https://github.com/pulp/pulp_file.git --branch "$PULP_FILE_BRANCH"
 if [ -n "$PULP_FILE_PR_NUMBER" ]; then
   cd pulp_file
   git fetch origin +refs/pull/$PULP_FILE_PR_NUMBER/merge

template_config.yml

@@ -2,7 +2,7 @@
 # were not present before running plugin-template have been added with their default values.
 
 additional_plugins:
-- pulp-certguard
+- { name: pulp-certguard, branch: master }
 black: false
 check_commit_message: true
 coverage: true
@@ -22,6 +22,7 @@ plugin_dash_short: pulpcore
 plugin_name: pulpcore
 plugin_snake: pulpcore
 pulp_settings: null
+pulpcore_branch: master
 pydocstyle: true
 pypi_username: pulp
 test_bindings: true