validate memory limits & error out if it's less than 524288

[unclejack]

Right now, we're not performing validation on the values passed as memory limits.
The following example shows what happens right now (it's not always the same, the printed errors vary):

docker run -i -m 512 ubuntu true
lxc-start: Device or resource busy - write /sys/fs/cgroup/memory/lxc/1b696c395503673593d80dd248439dbf15c1c68e6e8be0a83944bf2f0c4aea15/memory.limit_in_bytes : Device or resource busy
lxc-start: Error setting memory.limit_in_bytes to 512 for lxc/1b696c395503673593d80dd248439dbf15c1c68e6e8be0a83944bf2f0c4aea15

lxc-start: failed to setup the cgroups for '1b696c395503673593d80dd248439dbf15c1c68e6e8be0a83944bf2f0c4aea15'
lxc-start: failed to spawn '1b696c395503673593d80dd248439dbf15c1c68e6e8be0a83944bf2f0c4aea15'

# FAILED EXECUTION

Ideally, we'd want to catch this and throw an error - it's the best we can do. We can't make assumptions about the value we were given as a memory limit, so we have to set a minimum acceptable memory limit in bytes so that we can detect bad limits.

This PR makes a change in server.go so that we throw an error right away if the memory limit is set to less than 512 KB (524288).
This changes docker's behavior to this:

# let's make it fail
 docker run -i -m 16 ubuntu true
2013/06/14 19:59:20 Error: Memory limit must be given in bytes (minimum 524288 bytes)
# did we exit cleanly?
 echo $?
1
# oh, so it has to be in bytes
# let's try again
 docker run -i -m 16777216 ubuntu true
# did we exit cleanly?
 echo $?
0
# is the minimum limit real?
 docker run -i -m 524288 ubuntu true
 echo $?
0
 docker run -i -m 524287 ubuntu true
2013/06/14 20:02:48 Error: Memory limit must be given in bytes (minimum 524288 bytes)
# it looks like it is

[creack]

LGTM

[creack]

@unclejack Could you add a unit test?

[unclejack]

@creack I'll take care of it, sure.

[unclejack]

@creack I've added the test which checks that container creation fails when the memory limit is below the allowed limit.

[shykes]

LGTM