add support for multiple host auth when requesting db

This is a feature request, not a bug!

I talked about this on the #juju irc channel but allow me formally describe the feature request here. When a charm uses the pgsql interface and requests a database through 'set_database()' it would be interesting to have more options to pass through (hosts and users). In my specific use case I request a database in charm A (on host A) but host B also requires access to the database. My charm A provides the details to B (acts as a proxy) but postgresql itself only allows host A in his pg_hba.conf file while an entry for host B should also be made. Allow me to illustrate:

Charm/Host B (x.y.z.b) ---- (connected) ---- Charm/Host A (x.y.z.a) ---- (pgsql interface) ---- postgresql

Feature Request:

Something like this where a list of hosts could be passed as a parameter to the set_database()
```
hosts = ['x.y.z.b']
pgsql.set_database('mydb', hosts)

```

or a seperate function

```
hosts = ['x.y.z.b']
pgsql.set_database('mydb')
pgsql.allow_hosts(hosts)

```

So that an entry is added for the same database but for host x.y.z.a in the pg_hba.conf file.

Note that the extra_pg_auth option of the postgresql charm makes it possible but with the above feature no manual steps are needed anymore!

Thanks in advance.