Fix random deadlocks in MSSQL database

[potiuk]

Default isolation level for the MSSQL database is READ_COMMITTED
but its implementation in MSSQL might cause random deadlocks for
select queries - because by default MSSQL uses shared locks
for READ_COMMITTED transaction isolation that might cause
deadlocks with the UPDATE statements which create exclusive
locks.

This change switches the MSSQL DB used during tests to have
READ_COMMITTED_SNAPSHOTS enabled and it adds check at the Airflow
startup to make sure that if the MSSQL database is used, the
READ_COMMITTED_SNAPSHOTS is set and documentation is added to
describe it.

It also allows to revert removal of 2017 version of mssql which
was done in the faith that removal of 2017 will remove the
flaky random test failures.

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[potiuk]

cc: @aneesh-joseph

[potiuk]

@aneesh-joseph -> if you have setup MSSQL before, setting the "READ_COMMITTED_SNAPSHOT" database option is CRUCIAL to avoid deadlocks on read. It is apparently set on by default on Azure databases, but it is disabled by default on regular mssql databases. in 2.2.0 we are checking if the option is enabled for the airflow database and fail if not.