Re-add EnlargeStoragePool invocation

[schu]

While working on 924bb3b I did not
re-add the EnlargeStoragePool invocation and first time users
encountered disk space issues more often again ever since.

Also, add a note to the README and troubleshooting document to tell
users what to do.

Partly covers #281

[schu]

I should add a comment regarding: #281 (comment)

[schu]

Doesn't work on my system currently:

$ sudo ./kube-spawn up --kubernetes-version v1.10.5
Using config file "/etc/kube-spawn/config.yaml"
Copying files for cluster ...
Generating configuration files from templates ...
Cluster c1 created
making iptables FORWARD chain defaults to ACCEPT...
Failed to start cluster: not enough space on disk for 3 nodes, Each node needs about 1679 MB, so in total you'll need about 5037 MB available.

I do have enough free space I think:

/dev/loop0                15G  2.5G   13G  17% /var/lib/machines
/var/lib/machines.raw on /var/lib/machines type btrfs (rw,relatime,space_cache,subvolid=5,subvol=/)

@dongsupark any idea what could be wrong here?

[dongsupark]

@schu I guess, there's still a corner case not being caught in GetPoolSize().
I cannot reproduce it right away. Maybe I would need some time to debug it. Will try later.
Anyway that does not seem to be directly related to this PR. This PR looks good to me.

[dongsupark]

Will merge this PR, and add another PR to write more documents.