Bug 1800058: Ability to generate new local saml cert without deleting old one

This patch:
1) adds a button to make new cert/pem without deleting old one on
plugin config page
2) have both certificates listed in metadata
3) allow the deletion of old certificate and moving new one into its
place

behatnotneeded

Change-Id: I12a0f65582a0198cbb5d8b55fcf77cc677088ba0
Signed-off-by: Robert Lyon <robertl@catalyst.net.nz>