Fix bootstrapping from Git on CentOS7 when systemd is not running

[vutny]

What does this PR do?

If you'd like to build a CentOS 7 container/VM image containing Salt using docker build or Packer etc with bootstrap-salt.sh, it will fail to perform git installation type.

The main problem is in install_centos_git_post() function which executes /bin/systemctl daemon-reload at the very end. As a result, if systemd is not running (because we're building an image and there is no init system running on target), later logic in the script detects the return code not equal to 0 and aborts execution.

By this PR the post install functions will always return 0. Also, I've made some optimizations to execute /bin/systemctl only when it is necessary.

Previous Behavior

Git installation type fails when building OS images:

 *  INFO: Running install_centos_git_post()
 * DEBUG: The passed destination(/usr/lib/systemd/system/) is a directory
 * DEBUG: Full destination path is now: /usr/lib/systemd/system//salt-minion.service
 * DEBUG: Copying /tmp/git/salt/pkg/rpm/salt-minion.service to /usr/lib/systemd/system//salt-minion.service
Created symlink from /etc/systemd/system/multi-user.target.wants/salt-minion.service to /usr/lib/systemd/system/salt-minion.service.
Created symlink from /etc/systemd/system/multi-user.target.wants/salt-master.service to /usr/lib/systemd/system/salt-master.service.
 * DEBUG: The passed destination(/usr/lib/systemd/system/) is a directory
 * DEBUG: Full destination path is now: /usr/lib/systemd/system//salt-api.service
 * DEBUG: Copying /tmp/git/salt/pkg/rpm/salt-api.service to /usr/lib/systemd/system//salt-api.service
Failed to get D-Bus connection: Operation not permitted
 * ERROR: Failed to run install_centos_git_post()!!!
 * DEBUG: Cleaning up the Salt Temporary Git Repository
 * DEBUG: Removing the logging pipe /tmp/bootstrap-salt.logpipe
 * DEBUG: Killing logging pipe tee's with pid(s): 35

New Behavior

Git installation type succeeds.