Add version comparison functionality for package_version

[rtorrero]

This PR allows the package_version gatherer to be used to compare versions on the agent. This way, we can handle the complexity of comparing RPM package version strings to the OS using zypper versioncmp (see zypper versioncmp --help for more details).

Notice that this just adds to the existing functionality of requesting package version strings which should still work as it previously did. To request a version comparison, a package name together with a version string needs to be provided in the argument, separated by a coma (,).

The returned value for such requests is a FactValueInt, containing:

• A value of 0 if the provided version string matches the installed package version for the requested package
• A value of -1 if the provided version string is older that what's currently installed.
• A value of 1 if the provided version string is newer than what's currently installed.

Example on how to collect such facts:

factRequests := []entities.FactRequest{
{
	Name:     "corosync_same_version",
	Gatherer: "package_version",
	Argument: "corosync,2.4.5",
	CheckID:  "check1",
},
{
	Name:     "corosync",
	Gatherer: "package_version",
	Argument: "corosync",
	CheckID:  "check2",
},

For such requests, the returned values should look like:

[]entities.Fact{
{
	Name:    "corosync_same_version",
	Value:   &entities.FactValueInt{Value: 0},
	CheckID: "check3",
},
{
	Name:    "corosync",
	Value:   &entities.FactValueString{Value: "2.4.5"},
	CheckID: "check1",
},

Notice how the first fact request asks for corosync against 2.4.5, which will cause that an integer instead of a regular FactValueString with a version string. The second one, as it just asks for a package name will return a version string as it did before.

[arbulu89]

I see some redundant call to get the version of the package.
PD: I wouldn't add not related changes to the PR

[nelsonkopliku]

Thanks!

[arbulu89]

Thanks @rtorrero