software_manager: check_installed() should not raise CmdError

The check_installed() method returns True or False depending on
wether the given package is installed or not. It should shield
callers of that method from erros when dealing with the low
level package manager.

Signed-off-by: Cleber Rosa <crosa@redhat.com>
software_manager: fix check for a package and version that is not installed

When calling check_installed() and asking for a specific version
check, the current code misleadingly returns True.

The actual cause is that the output of the rpm command ("package
foo is not installed") is simply compared with the asked for version,
and the string comparison returns True.

For now just return False if that specific output is returned. A
more complete rewrite may be needed for fail proof version comparison.

Signed-off-by: Cleber Rosa <crosa@redhat.com>