qa/vstart_runner: store FSID in LocalContext instances

qa/vstart_runner: store FSID in LocalContext instances

1. Method cluster() in ceph.py creates a dictionary "ctx.ceph", attaches
   a namespace to ctx.ceph[cluster_name], create an attribute "fsid" and
   stores Ceph cluster's FSID in it.

2. The method kernel_mount.KernelMount._get_debug_dir() uses that "fsid"
   attribute to get Ceph cluster's FSID. (The exact that does that is
   "fsid = self.ctx.ceph[cluster_name].fsid").

3. Test test_readahead.TestReadahead.test_flush() crashes with
   vstart_runner.py because that test eventually calls _get_debug_dir()
   and "ctx" in case of vstart_runner.py doesn't hold "ceph" dictionary
   or anything similar.

Adding a dictionary, similar to the one added in ceph.py, to
vstart_runner.LocalContext's instances will fix this issue.

Fixes: https://tracker.ceph.com/issues/55694
Signed-off-by: Rishabh Dave <ridave@redhat.com>