check the RPM signature of zypper pkg.download packages and report errors

[meaksh]

What does this PR do?

• Create new method in rpm module which allow you to check if the signature of a RPM file is correct or not:

# salt '*' lowpkg.checksum /var/cache/zypp/packages/SLE-12-SP1-x86_64-Pool/x86_64/nmap-6.46-1.72.x86_64.rpm
minionsles12sp1-suma3pg.vagrant.local:
    ----------
    /var/cache/zypp/packages/SLE-12-SP1-x86_64-Pool/x86_64/nmap-6.46-1.72.x86_64.rpm:
        True

• Now zypper pkg.download checks if the packages are successfully downloaded and reports errors when some packages fail.

What issues does this PR fix or reference?

This PR fixes inconsistent behavior of zypper pkg.download as it currently doesn't report errors correctly and it doesn't check if the downloaded may be corrupted.

Before fix:

# salt '*' pkg.download nmap foo
minionsles12-suma3pg.vagrant.local:
    ----------
    nmap:
        ----------
        path:
            /var/cache/zypp/packages/SLE-12-x86_64-Pool/x86_64/nmap-6.46-1.72.x86_64.rpm
        repository-alias:
            SLE-12-x86_64-Pool
        repository-name:
            SLE-12-x86_64-Pool

After fix:

# salt '*' pkg.download nmap foo
minionsles12sp1-suma3pg.vagrant.local:
    ----------
    _error:
        The following package(s) failed to download: foo
    nmap:
        ----------
        path:
            /var/cache/zypp/packages/SLE-12-SP1-x86_64-Pool/x86_64/nmap-6.46-1.72.x86_64.rpm
        repository-alias:
            SLE-12-SP1-x86_64-Pool
        repository-name:
            SLE-12-SP1-x86_64-Pool

Tests written?

No

[isbm]

I am not sure I like this part of re-assigning the same var. I would do this way:

for package_file in paths:
    ret[package_file] = (bool(__salt__['file.file_exists'](package_file)) and 
                        not __salt__['cmd.retcode'](["rpm", "-K", "--quiet", package_file], 
                                                    ignore_retcode=True, 
                                                    output_loglevel='trace', 
                                                    python_shell=False))

What you think?

[meaksh]

You're right. I like your proposal, it doesn't break the flow as mine does.
I'm going to add it in a commit

[isbm]

@cachedout 🆗 in case you're wondering. 😉

[cachedout]

@isbm I was going to ask! You beat me to it. ;]

Thanks for this @meaksh . Good work here.

[meaksh]

Thank you @cachedout and @isbm for your help :)