Introduce function to check for utlilities

[gudmdharalds]

vip-go-ci uses various utilities, such as git, phpcs, as well as php. What is missing though is a function to check for the existence of such utilities, to ensure that they exist before using them, but in a way that is easy and consistent for developers to implement. The function should support relative and absolute paths.

The function should ideally be able to do such a check in the following ways:

vipgoci_util_exists( '/usr/bin/git', '--version', 'git version', 0 );

And:

vipgoci_util_exists( 'git', '--version', 'git version', 0 );

The parameters are:

• Path to execute
• Parameter to add to the command
• String to look for and expect in the output when executed
• Expected exit-status of the command

This way one can easily check for the existence of utilities as required. All checks should be performed in vipgoci_run().