cluster: always load cluster tags in load_receipt

Updated load_receipt so that the bail-out when load_volumes and
load_plugins are both False occurs *after* the cluster tags have been
loaded. This ensures at the very least the core cluster configuration
gets loaded.
cluster: fix filters in spot_requests property

Updated the VPC check in Cluster.spot_requests to make sure the nodes
were explicitly launched in a VPC by StarCluster. This is needed in
order to make sure the spot request filters are correct in the
Cluster.spot_requests property. This is because StarCluster launches VPC
nodes differently than non-VPC (in order to facilitate the public_ips
                option) which changes the filters needed in order to
correctly account for the instances. The previous check assumed that if
the cluster group had a vpc_id attribute then StarCluster must have
launched it, however, this is not true for the default VPC. In this case
StarCluster launches the clusters with a non-VPC configuration but the
default VPC automagically adds the group and instances to the default
VPC and subnet.

closes gh-377
sge: add new spots_per_host setting

Allows users to specify how many slots each exec host in the cluster has
by default *before* the exec host gets added to a queue and is available
to accept jobs.
sge: document new slots_per_host setting
Merge branch 'feature-sge-slots-per-host' into develop
sge: call _setup_sge_profile from within _inst_sge

The SGE profile setup is really part of installing SGE and belongs
within SGEPlugin._inst_sge
sge: remove redundant admin/submit host setup

All nodes are automatically added as submit and admin hosts by the SGE
installer script on initial cluster setup. Only newly added nodes (ie
from addnode) need to be explicitly added as submit and admin hosts.
sge: run qconf commands on exec host in _inst_sge

This avoids a threadpool issue where the same SSH connection object
can't be called by more than one thread. Each node in the cluster is an
admin host by default so the same qconf commands in _inst_sge can be
directly executed on the node being added as an exec host which avoids
the threadpool issue.
sge: fix parallel environment slot allocation

Updated the pe creation code to respect master_is_exec_host and
slots_per_host settings when computing the total number of slots for the
parallel environment.
addnode: fix help text
docs: convert github issue references to links
bump version to 0.95.4
update Changelog for 0.95.4 release
Merge branch 'hotfix-0.95.4' into develop
bump version back to development status (0.9999)
docs: update image host command with -I,-m options

Using these options instead of -i and -n prevents an issue where the
default cluster template in the config may override these values if
master_instance_type and master_image_id are defined. This ensures the
user gets exactly the instance type and base AMI they are expecting when
launching the image host regardless of their config.

closes gh-388
Add support for R3 instance types
Merge branch 'r3-instance-type-support' into develop

closes gh-392
closes gh-393
cluster: fix bug in _create_flat_rate_cluster

Only call Cluster.create_nodes if the launch map actually contains
aliases for the image/type. This is a regression introduced from the
changes in 0.95.4.
bump version to 0.95.5