account: delete filesystem traces of an account

This patch adds a new virtual API to Account, flush(),
that does nothing by default. Concrete classes may overload it
when it's needed to remove filesystem entries created by them.

Then this patch adds also overloads of this method in SIPAccountBase and
RingAccount to delete all files added by these account classes.

Finaly, the removeAccount() external API calls flush by default, but
internal calls do not, to keep a conservative behaviour
and not destroying account files due to yml parsing errors.

Change-Id: I52c4e225f4d41f0e3d74f63838e56a769011b58e
Tuleap: #988