Add function to wait for pkgdb lock to clear

Arch pkg tests that call pkg.latest_version seem to fail on running
this function due to the package database still being locked from
previous tests. This adds a function that waits (in 5 second increments)
for up to 60 seconds for the pkgdb to be unlocked.

http://jenkins.saltstack.com/job/salt-rs-arch/474/artifact/salt-runtests.log/*view*/