php: simplify installation

build.yaml

config.m4

package.xml

templates/config.m4.template

templates/package.xml.template

test/distrib/php/distribtest.php

test/distrib/php/run_distrib_test.sh

tools/dockerfile/distribtest/php_jessie_x64/Dockerfile

tools/dockerfile/grpc_artifact_linux_x64/Dockerfile

tools/run_tests/artifact_targets.py

tools/run_tests/build_artifact_php.sh

tools/run_tests/build_package_php.sh

tools/run_tests/distribtest_targets.py

tools/run_tests/package_targets.py