make get-pip.py download failures more clear

curl dying ends up being a really unclear failure condition, and
hard to fingerprint in the gate. We should make this much more
explicit when we die.

Also, don't trust the upstream filename, because all the rest of
our logic would break if it changes anyway.

Change-Id: Ibc2a96b33471d24c597af0d7af896fb10523156f
Update again and retry a failed package install

On ubuntu, if we run into an error installing the package, it can be
because of transient upstream errors with repo sync state. Although
retrying the update won't fix it all of the time, it's low cost enough
to be worth a try before we totally give up.

Related-bug: 1286818
Change-Id: I522ac0d0bd8f82dc98f386c89f66c2b743efa525
Fix Apache site config handling on Fedora

Allow enable/disable_apache_sites() on Fedora to gracefully fail if the
config is not present.  This is primarily an issue when the config from
a previous run is not completely cleaned out (unstack.sh vs clean.sh).

Make APACHE_CONFIG_DIR fully qualified and overrideable in local.conf.

Also fix Horizon's handling of its Apache config file to be removed
in clean.sh.

Change-Id: I78a5de579dd3b02fa2e4e7e00ac0aabe71b531ad
Merge "Fix Apache site config handling on Fedora"
Merge "Update again and retry a failed package install"
Merge "make get-pip.py download failures more clear"