ipa-server-install: Make temporary pin files available for the whole installation

We names of files with pkcs12 pins to installers which may continue to
use the files after the initial call to create_instance, at which point
the installer has already removed them.
Also, some of the files were not properly removed on failure.

Use ipautil.write_tmp_file for the pin files, which returns a
NamedTemporaryFile object that removes the underlying file when it is
garbage-collected.
Create the files at start of installation. This will allow checking
the pkcs#12 files before the system is modified.
ipa-server-install: Remove the --selfsign option

Instead, certificates in pkcs12 files can be given to set up
IPA with no CA at all.
Use a flag, setup_ca, to signal if a CA is being installed.

Design: http://freeipa.org/page/V3/Drop_selfsign
Part of the work for: https://fedorahosted.org/freeipa/ticket/3494
ipa-server-install: correct help text for --external_{cert,ca}_file

The options take PEM certificates, not PKCS#10.
This corrects both the --help output and the man page.

https://fedorahosted.org/freeipa/ticket/3523
Remove unused ipapython.certdb.CertDB class
ipaserver.install.certs: Introduce NSSDatabase as a more generic certutil wrapper

The CertDB class was meant to be a wrapper around NSS databases,
certutil, pk12util, etc. Unfortunately, over time it grew too
dependent on the particular scenarios it is used in.
Introduce a new class that has no knowledge about IPA configuration,
and move generic code to it.

In the future, generic code should be moved to NSSDatabase, code
for the self-signed CA should be removed, and IPA-specific code may
stay in CertDB (which calls NSSDatabase).
Trust CAs from PKCS#12 files even if they don't have Friendly Names

Instead of trusting all certificates with friendly names,
now all certs without a "u" flag are trusted as root certs.
dsinstance, httpinstance: Don't hardcode 'Server-Cert'
Support installing with custom SSL certs, without a CA

Design: http://freeipa.org/page/V3/CA-less_install
https://fedorahosted.org/freeipa/ticket/3363
Load the CA cert into server NSS databases

The CA cert was not loaded, so if it was missing from the PKCS#12 file,
installation would fail.
Pass the cert filename to the server installers and include it in
the NSS DB.

Part of the work for: https://fedorahosted.org/freeipa/ticket/3363