Install full system Python 3 for testing

Travis has changed their pyenv configuration such that the default
Python comes first in the PATH, instead of a custom Python 3.5.
It's a good idea to use the system Python for a few reasons:
  - pyenv is not used on macOS, and we don't have it inside Docker
  - This makes it easier to run on stock hosts without pyenv,
    which is useful for running tests on e.g. our real build machines

Unfortunately, the Ubuntu images do not have the necessary python3
virtualenv package installed, and the name differs between Ubuntu
versions, so these configs require some additional snowflake steps.

Use the Salt python SLS file to ensure that the system Python is
configured appropriately before running the test suite,
which allows encapsulating the complexity into the Salt code for reuse.
Make sure to install Salt first if it is not already installed.

.travis/dispatch.sh

@@ -23,16 +23,21 @@ travis_fold_end() {
     printf "travis_fold:end:%s\n" "${1}"
 }
 
-run_salt() {
+install_salt() {
     travis_fold_start "salt.install.$1" 'Installing and configuring Salt'
     .travis/install_salt.sh -F -c .travis -- "${TRAVIS_OS_NAME}"
     travis_fold_end "salt.install.$1"
+}
+
+run_salt() {
+    install_salt "${1}"
 
     travis_fold_start "grains.items.$1" 'Printing Salt grains for debugging'
     salt_call grains.items
     travis_fold_end "grains.items.$1"
 
-    travis_fold_start "state.show_highstate.$1" 'Performing basic YAML and Jinja validation'
+    travis_fold_start "state.show_highstate.$1" \
+        'Performing basic YAML and Jinja validation'
     salt_call --retcode-passthrough state.show_highstate
     travis_fold_end "state.show_highstate.$1"
 
@@ -58,16 +63,30 @@ run_inside_docker() {
 }
 
 
-setup_venv() {
-    local -r VENV_DIR="/tmp/saltfs-venv3"
+setup_test_venv() {
+    if ! which salt-call >/dev/null; then
+        install_salt 'test_venv'
+    fi
+    travis_fold_start 'test_venv.install_python3' \
+        'Setting up Python 3 virtualenv for testing'
+    # Use the system Python 3 to make it easy to run tests on fresh hosts
+    # Make sure dependencies are installed (like `python3-venv` on Debian derivatives)
+    salt_call --retcode-passthrough state.sls python
+    travis_fold_end 'test_venv.install_python3'
+
+
     printf "Using %s at %s\n" "$(python3 --version)" "$(which python3)"
 
+    travis_fold_start 'test_venv.install_requirements' \
+        'Installing pip dependencies for testing'
+    local -r VENV_DIR="/tmp/saltfs-venv3"
     python3 -m venv "${VENV_DIR}"
     set +o nounset
     source "${VENV_DIR}/bin/activate"
     set -o nounset
     pip install wheel
     pip install -r requirements.txt
+    travis_fold_end 'test_venv.install_requirements'
 }
 
 
@@ -79,7 +98,7 @@ fi
 
 if [[ "${SALT_NODE_ID}" == "test" ]]; then
     # Run test suite separately for parallelism
-    setup_venv
+    setup_test_venv
     ./test.py
 elif [[ -n "${SALT_DOCKER_IMAGE:-}" ]]; then  # macOS bash is too old for `-v`
     printf "Using %s\n" "$(docker -v)"
@@ -108,7 +127,7 @@ else
     fi
 
     # Only run tests against the new configuration
-    setup_venv
+    setup_test_venv
 
     # TODO: don't hard-code this
     if [[ "${SALT_NODE_ID}" == "servo-master1" ]]; then