Update WP-CLI to 1.5.1

Update from `1.6.1` -> `1.7.0` which addresses roots#943 ([DEPRECATION WARNING]: The use of 'include' for tasks has been deprecated.)

Avoids deprecation warnings introduced in Ansible 2.4:
"The use of 'include' for tasks has been deprecated."

Add xdebug.remote_autostart to simplify xdebug sessions

Because `git-core` is now a dummy package of `git`.

See: http://git.661346.n2.nabble.com/git-core-vs-git-package-on-ubuntu-tp7576083p7576085.html

Common: Install `git` instead of `git-core`

The X-Frame-Options header has been obsoleted by the frame-ancestors
directive. Retain the X-Frame-Options header for older browsers.

Return empty X-Frame-Options header for WordPress Customizer content
to prevent the conflict that SAMEORIGIN would have with the ALLOW-FROM
option that WordPress adds on its own (Safari browser).
Discussion in https://core.trac.wordpress.org/ticket/40020

In rare cases the wp_installed registered var may be missing the
stderr attribute, so add a default to avoid related error.

The `wp core  is-installed` command return code is 1 if WP is simply
not installed. However, in rare cases the command may return some
other return code indicative of true failure, so fail if rc > 1.

- Exit with `127` when not enough arguments
- Exit with `1` when hosts file not exist

See: http://www.tldp.org/LDP/abs/html/exitcodes.html

* Bump Ansible version_tested_max to 2.5.3

Convert Jinja2 tests from filter format to `var is testname` format.
Encourage users on Ansible 2.5.0 to upgrade to avoid erroneous warnings
fixed in ansible/ansible 37538

Enabled by default

Quotes prevent Ansible from interpolating the variable value as True.
True is an invalid value for fastcgi_cache_background_update and would
would make Nginx unable to reload.

Verify `wp-cli.phar` checksum