mxisd supports several identity stores. Add support to configure two of them:
* synapseSql (storing identities directly in Synapse's database)
* LDAP
This removed the need to copy `mxisd.yaml.j2` to the inventory in case one wants
to use LDAP as identity store. Note that the previous solution (copying
`mxisd.yaml.j2` was poor because of two reasons:
* The copy remains outdated in case the original is updated in future versions
of this repo.
* The role's configuration should be in one place (configured only through role
variables) instead of in multiple.
Configuring more identity stores through role variables can be supported in the
future.
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